SQL Server's datetime handling is a little iffy (at best), but it can be done. Unlike Oracle's TRUNC() function which chops off the time portion for evaluation purposes, you have to reset all time values to midnight (momentarily, not permanently) to get a solid comparison in SQL Server:

CAST ( CONVERT ( VARCHAR, datetime_field, 101 ) AS DATETIME )

This will convert datetime_field into a varchar of format mm/dd/yyyy then cast it back into a datetime value (mm/dd/yyyy 00:00:00.000) for comparison.

Hope this helps.