4.1 jailbroken with redsn0w_win_0.9.5b5-5
I'm having the exact same problem. I updated my iPhone 3G to OS 4.1 last night on Windows XP and then jailbroke it with the latest redsn0w beta 5-5 and that worked fine, but I went to sync the phone with iTunes later and it tried to sync but it failed because it was unable to make a connection with my phone.
I can plug it in to charge it, and the phone recognizes that it's plugged in, and so does iTunes, but no message pops up on the phone display saying its trying to sync. I also can't restore it to factory settings or to a previous .ipsw. When I hold the sleep button until the "slide to power off" message pops up and I slide it to the right, the phone initially tries to turn off, but the circle thing on the phone stops spinning and it goes back to my lock screen.
I cannot manually put the phone in DFU or Recover Mode either, and I know how; the phone just won't do it. I basically have no way of restoring the phone. I haven't been able to find anything conclusive online about how to fix this; I even restarted my phone and PC several times and removed everything Apple-related from my PC and re-downloaded iTunes and tried again, but it still won't make the connection with my phone.
If you hear anything about how to fix his problem, please repost it here.