DATEDIFF(hour, start_date, end_date) will give you the number of hour boundaries crossed between start_date and end_date.
If you need the number of fractional hours, you can use DATEDIFF at a higher resolution and divide the result:
DATEDIFF(second, start_date, end_date) / 3600.0
The documentation for DATEDIFF is available on MSDN:
http://msdn.microsoft.com/en-us/library/ms189794%28SQL.105%29.aspx