if you messed up an NV item you will need to get into the radio bootloader to fix it. That would be like dcd said with the rtask commands, but to get there you will need a MFG spl such as Version .25 from this thread:
http://forum.ppcgeeks.com/showthread.php?t=42503
Now here's the problem though. Since you can't boot to Windows mobile you can't load that SPL as the loader needs access to ram which isn't allowed with the .40 SPL if i remember correctly. Since it's a stock SPL i really think warranty might be the best option at this point.
Oh, yeah one last thing to try:
http://forum.ppcgeeks.com/showthread.php?t=9229
The stuck in bootloader tool sometimes works even if flashing a rom exe hasn't for some reason. It's at least worth a shot. This can also be done with mtty by entering "set 16 0" as hinted at in the description of the app in that thread.