Just loaded up iOS 5 beta 6 (was on 4.3.5) and the matching iTunes 10 (v10.5b90). So far, its showing problems where Podcasts are sitting on a Samba network drive. In all honesty, I had some level of slowness before doing this, but never actually had any prevention of syncs, but THIS version really doesn't want me to sync after restoring my phone to defaults.
So thanks to some stracing of Samba, I found an aweful lot of reading of ._xyz files inside the Podcast directories. I assume these are some sort of metadata files for each individual podcast file but for whatever reason until I got rid of them (moved them to a new folder 'old' in each subdir) the damned iTunes just would hang forever. I didn't look far enough into it, but these files are only 4K each and I left iTunes and my phone OVERNIGHT to sync and it was still going. Getting rid of those files and tada, got into the Podcast tab again finally.
Now, I'm not sure what iTunes is doing when it is 'determining tracks to sync' but I can tell it's doing an occasional read of iTunes/iTunes Library.xml and definitely is also having troubles with the Podcasts folder. Once I renamed the folder, I was finally able to reach step 5 of 5 and start syncing my ringtones and apps again.