For those of you that have a windows machine, you can now use GreenPois0n to jailbreak iOS 4.2.1 untethered. The GreenPois0n.com website is still overloaded, so you can download it via this mirror.
You can also use the mac video tutorial to update your iPhone 4 to 4.2.1 and preserver your 1.59.00 baseband for the UltraSn0w unlock.
Just did the greenpois0n 4.2.1 jailbreak last night. Went smooth the first time, runs great so far with no hiccups. ?