with CTE_R as
(
    select
        t.*,
        ROW_NUMBER() OVER(PARTITION BY UserID ORDER BY LOGTIME) as RowNum
    from @MyTable as t
)

select 
    c.UserID,
    c.LOGTIME
from CTE_R as c
left join CTE_R as a
    on a.UserID = c.UserID and
       a.RowNum = c.RowNum - 1
where DATEDIFF(MINUTE, a.LOGTIME, c.LOGTIME) > 10 or
      c.RowNum = 1

UserID          LogTime
U1                20111225 14:20:00
U1                20111225 14:28:00
U1                20111225 14:30:00
U1                20111225 14:31:00
U1                20111225 14:40:00
U1                20111225 14:48:00
U1                20111225 14:51:00
U2                20111225 14:20:00
U2                20111225 14:28:00
U2                20111225 14:33:00
U2                20111225 14:43:00
U2                20111225 14:53:00
U2                20111225 14:58:00
U2                20111225 15:09:00