CREATE PROCEDURE dbo.GetGroupedIntervals
    @MinuteInterval TINYINT = 1
AS
BEGIN
    SET NOCOUNT ON;

    DECLARE 
        @IntervalCount INT, @StartDate SMALLDATETIME;

    SELECT
        @StartDate = DATEADD(MINUTE, -1, MIN(dt)), 
        @IntervalCount = (DATEDIFF(MINUTE, MIN(dt), MAX(dt)) 
            + @MinuteInterval) / @MinuteInterval
          FROM dbo.SomeTable -- WHERE ...;

    ;WITH dates(s,e) AS
    (
        SELECT 
            DATEADD(MINUTE, @MinuteInterval*(n.n-1), @StartDate),
            DATEADD(MINUTE, @MinuteInterval*(n.n), @StartDate)
        FROM
        (
          SELECT 
            TOP (@IntervalCount) ROW_NUMBER() OVER (ORDER BY o.[object_id])
            FROM sys.all_objects AS o, sys.all_columns
            ORDER BY o.[object_id]
        ) AS n(n)
    )
    SELECT StartDate = d.s, c = COUNT(s.dt) 
    FROM dates AS d
    LEFT OUTER JOIN dbo.SomeTable AS s
        ON s.dt >= d.s AND s.dt < d.e
        -- AND any filter criteria for dbo.SomeTable?
    GROUP BY d.s
    ORDER BY d.s;
END
GO

• I go down a minute in case the start date was rounded up when converted to SMALLDATETIME. For the 1 minute interval and possibly others, this can lead to 0 counts for the first interval. You can adjust how this rounding happens (e.g. you can use FLOOR() to make sure it always rounds down). All depends on how accurate you need to be.
• I didn't include any WHERE clause but you may need to have those to filter. For example your query may want all the intervals for a given day. You may want to change the calculation of dates to produce all the intervals for the day, rather than all the intervals between the min and max time found in dbo.SomeTable on that day.
•   The join using s.dt is likely to perform better than any extraction methods using datepart if the datetime column in your base table has an index (or might in the future).

CREATE PROCEDURE dbo.GetGroupedIntervals
    @MinuteInterval TINYINT = 1
AS
BEGIN
    SET NOCOUNT ON;

    DECLARE 
        @IntervalCount INT, @StartDate SMALLDATETIME;

    SELECT
        @StartDate = DATEADD(MINUTE, -1, MIN(dt)), 
        @IntervalCount = (DATEDIFF(MINUTE, MIN(dt), MAX(dt)) 
            + @MinuteInterval) / @MinuteInterval
          FROM dbo.SomeTable -- WHERE ...;

    ;WITH dates(s,e) AS
    (
        SELECT 
            DATEADD(MINUTE, @MinuteInterval*(n.n-1), @StartDate),
            DATEADD(MINUTE, @MinuteInterval*(n.n), @StartDate)
        FROM
        (
          SELECT 
            TOP (@IntervalCount) ROW_NUMBER() OVER (ORDER BY o.[object_id])
            FROM sys.all_objects AS o CROSS JOIN sys.all_columns AS c
            ORDER BY o.[object_id]
        ) AS n(n)
    )
    SELECT StartDate = d.s, c = COUNT(s.dt) 
    FROM dates AS d
    LEFT OUTER JOIN dbo.SomeTable AS s
        ON s.dt >= d.s AND s.dt < d.e
        -- AND any filter criteria for dbo.SomeTable?
    GROUP BY d.s
    ORDER BY d.s;
END
GO

• The join using s.dt is likely to perform better than any extraction methods using datepart if the datetime column in your base table has an index (or might in the future).
• I assumed you wanted to show all the intervals within the range. If you don't want to show the intervals with 0 counts, just change the left outer join to an inner join.
• I go down a minute in case the start date was rounded up when converted to SMALLDATETIME. For the 1 minute interval and possibly others, this can lead to 0 counts for the first interval. You can adjust how this rounding happens (e.g. you can use FLOOR() to make sure it always rounds down). All depends on how accurate you need to be.
• I didn't include any WHERE clause but you may need to have those to filter. For example your query may want all the intervals for a given day. You may want to change the calculation of dates to produce all the intervals for the day, rather than all the intervals between the min and max time found in dbo.SomeTable on that day. The following variation takes care of this, by presenting the data for a single day starting from midnight and incrementing by @MinutInterval:

...

CREATE PROCEDURE dbo.GetGroupedIntervalsByDay
    @Date           DATE,
    @MinuteInterval TINYINT = 1
AS
BEGIN
    SET NOCOUNT ON;

    DECLARE 
        @IntervalCount INT,
        @StartDate     SMALLDATETIME = @Date;

    SELECT
        @IntervalCount = 1440 / @MinuteInterval;

    ;WITH dates(s,e) AS
    (
        SELECT 
            DATEADD(MINUTE, @MinuteInterval*(n.n-1), @StartDate),
            DATEADD(MINUTE, @MinuteInterval*(n.n),   @StartDate)
        FROM
        (
          SELECT 
            TOP (@IntervalCount) ROW_NUMBER() OVER (ORDER BY o.[object_id])
            FROM sys.all_columns AS o
            ORDER BY o.[object_id]
        ) AS n(n)
    )
    SELECT StartDate = d.s, c = COUNT(s.dt) 
    FROM dates AS d
    LEFT OUTER JOIN dbo.SomeTable AS s
        ON s.dt >= d.s AND s.dt < d.e
        -- AND any filter criteria for dbo.SomeTable?
    GROUP BY d.s
    ORDER BY d.s;
END
GO

Sample calls:

EXEC dbo.GetGroupedIntervalsByDay @Date = '20120516', @MinuteInterval = 1;
EXEC dbo.GetGroupedIntervalsByDay @Date = '20120516', @MinuteInterval = 2;
EXEC dbo.GetGroupedIntervalsByDay @Date = '20120516', @MinuteInterval = 5;

Abridged results from the last call:

StartDate             c
-------------------   ----
2012-05-16 00:00:00   0
2012-05-16 00:05:00   0
2012-05-16 00:10:00   0
...
2012-05-16 12:40:00   0
2012-05-16 12:45:00   0
2012-05-16 12:50:00   45
2012-05-16 12:55:00   53
2012-05-16 13:00:00   16
2012-05-16 13:05:00   55
2012-05-16 13:10:00   111
2012-05-16 13:15:00   0
2012-05-16 13:20:00   0
...
2012-05-16 23:45:00   0
2012-05-16 23:50:00   0
2012-05-16 23:55:00   0

(Again if you don't want to include the intervals with no counts, change the left outer join to an inner join. You also might find some odd results if you choose an interval that doesn't fit nicely inside of 1440. I'll leave that case as an exercise to the reader.)