Same is happening for me, but on Linux.
Polling occurs on schedule, but no changes are picked up. There seems to be no difference whether it's on a branch or master, as these two jobs show:
Started on Feb 3, 2010 3:01:24 PM
[poll] Last Build : #876
[poll] Last Built Revision: Revision 1931b23f384c5006eaf9f6ce566332432c71787a (origin/master)
Started on Feb 3, 2010 3:06:24 PM
[poll] Last Build : #74
[poll] Last Built Revision: Revision dee124a5b2dcb8c956cc7dcfc7e794ddbdae0dc7 (origin/branch-1.0)
In both cases, there have been new commits throughout the day which aren't being recognised.
Building manually still works as expected.