Hey I'm the guy who was looking into the fullscreen problem on 10.5 (and the EXIF bug which might be what >>161 is talking about. It sounds similar). I decided to find a good solution to the fullscreen and I managed to get one and it works great so far.

I've got a patch against what's currently in SVN that includes the fullscreen fix and the EXIF "fix" (it fixes the problem that I see but I'm not sure it's the most graceful solution). There's a slight problem though that the current revision (r17) does not compile because it references some files and frameworks that are not there:

XeePhotoshopPICTLoader.h
XeeExperimentalImage1Saver.h
XADMaster
UniversalDetector

my patch doesn't include the removal of references to these files and frameworks that I needed to make it build (I'm assuming everything builds fine for you). If you want the patch with these extra changes, let me know.

Meanwhile I've got a Universal build you can try out here:
http://theidiotproject.com/Code/Xee+patch.app.zip
and the patch file here:
http://theidiotproject.com/Code/fullscreen+exif.patch

It's probably not a perfect solution to either problems, but they do work just fine in my tests. The EXIF one is particularly nasty because it can cause your whole machine to grind to a halt while it loops. If nothing else I hope they help.