PwnageTool 4.2 can also preserve iPhone baseband that means you unlock your iDevice with UltraSn0w.
According to iPhone Dev Team blog:
“What’s in a name? Well in the case of an HFS volume name on iOS, an untether exploit — as the Chronic Dev Team revealed last week with an untether for the 4.2.1 jailbreak, which had previously been a tethered JB for most recent devices since 4.2.1’s release in November. With their permission, we’ve incorporated their 4.2.1 “feedface” untether into today’s PwnageTool 4.2. This means iPhone unlockers can safely restore to a custom iOS 4.2.1 pre-jailbroken IPSW and retain their current baseband and unlock. PwnageTool also supports all the other 4.2.1 devices other than iPod touch 2G:
PwnageTool also includes two very recent improvements to the 4.2.1 JB: iBooks was just fixed by @comex and @pushfix last night so that it works as intended on DRMed books, and the wifi problem on AppleTV 2G was fixed by @nitotv, @DHowett, and @saurik. Both of these fixes will also be available in upcoming Cydia package updates, so if you’re already jailbroken you can wait for those updates rather than restore and jailbreak again.
The various components to the 4.2.1 untether (including a second exploit involving Mach-o headers) were worked out by 0naj, posixninja, and pod2g, and a nice writeup by 0naj is available on the wiki. The actual injection method uses geohot’s limerain exploit for most devices. And even though 4.3 is just around the corner, the exploit used has already been closed in the latest 4.3 betas, so it made sense for the 4.2.1 untether to be released when it was. It also appears that a security researcher named @i0n1c has a 4.3 untether ready for when Apple releases the final 4.3 FW, so it may not be a long wait at all with 4.3!”
At the moment PwnageTool 4.2 is only available for Mac users.