I have x51v and several days ago when I try to activesync my computer reboots after flashing a message 'retreiving settings' . this happens with both 4.2 and 4.5.
I had upgraded PocketPlus and installed the latest Dell patch before this happened but not sure this caused it. I think there were some Windows SP updates also but those are hard to keep track of.

I have my PC set up to dual boot into Vista and there the Mobile center is syncing just fine.

I have removed the connection and then reestablished it Didn't help
I have uninstalled both versions and reinstalled them one at a time Didn't help
I deleted the Dell patch Didn't help

Not sure what to try now!!