The issue with monitoring my packages for new versions just cleared out: I got a PPA.

To use it just add the following line to your sources.list. Hint: You can do it graphically using System->Admin->Software Sources.

deb http://ppa.launchpad.net/madman2k/ubuntu gutsy main restricted universe multiverse

Currently you will only find pidgin2.2.0 there, but I will merge my packages by and by.

PPA are great idea, since this helps non-MOTU packagers like me a lot; no free webspace searching, automatically building for amd64 and better handling for Users.

The idea is not that new though; Arch Linux had its TUR already for quite a while.

I'm using your repository for GIMP updates; 2.4.3 is there today, but the Update Manager won't let me install them. Is there a way I can force the upgrade?

maybe it was not built yet. does the problem still exist?

Yes. I get the following message:

Error authenticating some packages

It was not possible to authenticate some packages. This may be a transient network problem. You may want to try again later. See below for a list of unauthenticated packages.

hmm, that cause for this might be that the PPA packages are not signed and thus cant be authenticated.
Usually you should only get a warning but still be able to install them. If this is a synaptic error, you could try to do the update with apt-get.
But I dont have a clue which option might prevent you from installing...

When I try through apt-get I get the message:

The following packages have been kept back:
gimp gimp-data gimp-python

Resolved it this morning; it was only in the Update Manager I was having the problem; when I went into Synaptic I was able to upgrade.