Give it a try and let me know if you agree… Quick test:
$ batch now
> ls
[control-d]
job 1 at Wed May 14 14:02:27 2008

$ atq
778240 Fri Jan 18 09:03:00 2002

———-

$ batch 17:00
> ls
[control-d]
job 5 at Wed May 14 17:00:00 2008
$ atq
778240 Sat Jul 17 12:39:44 1993

Update: 10.5.3
This doesn’t seem to happen anymore in 10.5.3, but it doesn’t quite seem to work either:

$ at 3:00
> touch /TEST.TEST
job 10 at Sun Jun 1 03:00:00 2008

This was ran at 2:59:30(or so) and as of 3:45:
$ atq
10 Sun Jun 1 03:00:00 2008

Yet, the command did not execute.

Update 9/21/09:

I don’t know how I missed this before, but in the man page for at on OS X, there is the following comment:

Note that at is implemented through the launchd(8) daemon periodically
invoking atrun(8), which is disabled by default.  See atrun(8)
for information about enabling atrun.

Going over to the atrun page shows that you can enable a launch daemon to periodically run queued at and batch jobs. The executiable that does this should be run with root permissions and is located at /usr/libexec/atrun