apt doesn't upgrade packages

[Aaron Tomb <atomb@ucsc.edu>]

I'm having a very strange problem. As of about a week ago, apt-get stopped
noticing new packages and upgrading them. It still downloads package lists, and
if I tell it a specific package version, as in:

    apt-get install mozilla-firefox=0.9.1-5

it'll install the newer version. However, if I just say 'apt-get upgrade' (or
dist-upgrade, or dselect-upgrade), it says that there are no new packages. I'm
using apt 0.5.26, dpkg 1.10.23, and dselect 1.10.23. No one else I've talked to
is having this problem, so I think it's an issue of configuration, rather than
a bug in apt. But I've had no luck trying to fix it. I've changed my
sources.list several times, to try different mirrors, to no avail. Does anyone
have any idea what might be wrong?

Thanks,
Aaron