Here is an interesting technique-- I leveraged the startswith procedure as implemented with Django on MySQL to achieve the result of only looking up a datetime through only the date. Basically, when Django does the lookup in the database it has to do a string conversion for the DATETIME MySQL storage object, so you can filter on that, leaving out the timestamp portion of the date-- that way %LIKE% matches only the date object and you'll get every timestamp for the given date.
datetime_filter = datetime(2009, 8, 22)
MyObject.objects.filter(datetime_attr__startswith=datetime_filter.date())
This will perform the following query:
SELECT (values) FROM myapp_my_object \
WHERE myapp_my_object.datetime_attr LIKE BINARY 2009-08-22%
The LIKE BINARY in this case will match everything for the date, no matter the timestamp. Including values like:
+---------------------+
| datetime_attr |
+---------------------+
| 2009-08-22 11:05:08 |
+---------------------+
Hopefully this helps everyone until Django comes out with a solution!