Finally the wait is going to over. From different reliable sources it is now confirmed that Apple will release iOS 4.2 shortly at 10 a.m Pacific Time for all iPhone and iPod Touch models except the original iPhone and iPod Touch first generation.
If you rely on unlock, you should keep your idevice away from this iOS 4.2 release because it will update the baseband too that is not unlockable yet.
There are also news that iPad will get iOS 4.2.1 (same iOS 4.2 version for iPad too) version which will bring some fixes related to WiFi issues.
Please make it sure that you have saved the SHSH blobs and backed up your idevice before going for update.
Update: According to some reports iOS 4.2.1 is now available via iTunes in some regions.