Its here friends, as the baseband unlock for 4.1 is not released yet, if you want to update your 4.0.x without updating your baseband (and thus preserving the unlock), you can follow instructions posted HERE.
Proceed at your own risk. If you don’t have SHSH saved, do not even try!
BUT, BUT , BUT Keep in mind that you currently won’t be able to jailbreak (and therefore unlock) your iPhone 4 /3GS on iOS 4.1, even though, you preserved the original baseband.
Good news it that, there has been reports that the bootrom exploit will be available soon.