well, have you tried installing any apps from cydia? i've herd intelliscreen has been causing problems on the boot.... if you have installed openssh to your phone your best bet is to ftp to your phone and try to delete those intelliscreen files... there are two additional files installed other than the app in /applications.
also "/Library/Intelliborn/"
and... well there is one more folder..... i think i coreservices... but i cant seem to find it.
Hopefully that is what your problem is and hopefully you can delete those files... if not, you are at least lucky enough to have itunes detecting it... so, you may just have to do a restore.. if you just pwned it 2 days ago, than that means you are not that far into customization... and a restore isnt that bad....
Hope this helps... if it does, dont forget to hit that button
*EDIT* Oh yeah, just remembered... i had the same problem the first time i pwned 2.1 too.... it was because i used pwnagetool and tried to sync in itunes... It booted just fine the first time, but after a sync with itunes i got a 1600 error and never booted back up, just hung there at the pineapple. Use quikpwn, BYpasses itunes completely... no problems since!
Let me know if this helps.