The iPhone Dev Team has released an updated version of ultrasn0w that adds support for iOS 4.3.2. Ultrasn0w 1.2.2 doesn’t support the latest iPhone baseband, which means you cannot use it to unlock phones using the latest baseband. But it does fix iOS 4.3.2 compatibility issues with phones with older basebands.
In plain English: If you have a phone that you’ve already carrier unlocked using older tools and you’ve been holding off on updating to iOS 4.3.2 so that you don’t lose the ability to make phone calls, it’s now safe to update. On the other hand if you were thinking of jailbreaking your brand new AT&T iPhone 4 or iPhone 3GS to use with T-Mobile or another carrier, ultrasn0w probably can’t help you yet.
The folks at Redmond Pie have put together a step-by-step tutorial for jailbreaking an iPhone 3GS or iPhone 4, preserving the old baseband, and using ultrasn0w to unlock your phone.