This is not a problem with Spaces, Expose, or multiple monitors.
I have a Mac Pro, fresh install of Leopard 10.5.2, fresh install of Adium. I set the contact list to hide on screen edges when Adium is in the background, and it worked... until I rebooted. Now, the contact list stays on the edge, and nothing will bring it back.
Adium seems to think the contact list is at its proper place. Command-slash makes that one-pixel-wode window disappear and return; Expose has no effect. If I drag it back on screen, the proper behavior returns until the next reboot.
I've made sure I'm docking it slightly to the left of the edge, not the right. I'm not sure what else to do for troubleshooting purposes.
Setup:
Mac Pro (early 2008), Xeon 8-core 3.2GHz
Leopard (non-server) 10.5.2, fresh install
1920x1200 Samsung monitor (SINGLE display) via DVI
16 GB RAM (that doesn't affect this; I just like saying it)
4x 300GB SAS Cheetah 15K.5 in RAID-10 via Areca 1680LP (ditto)
Spaces not (yet) enabled