Here's what works and don't work for me:

Works:
Opera (with the button change in the "Go To" menu
Total Commander
TCPMP
PMVault
MortPlayer
XnView Pocket
Resco Radio (storage card)
Omega 1-Calc
iGuidanceQVGA (storage card) (from a whopping 2.36MB to a slim 694kb!)
PocketPlus (partially - "PlusTodayPluginMFC.dll" messes up the items menu. "PlusAppHook.dll" is fine to compress)

Overall, went from about 8.x MB free in storage to a healthy 14.6 MB
That's not including iGuidance and Resco Radio, which resides in my storage card!

What doesn't work
Today Agenda (popup menu's all messed up)
Tweaks2k2 (Can't compress - .Net exe not supported)
PocketPlus (read above)