This is something I noticed earlier today when I more or less bricked my phone!!!
The newest version of ROM Manager wants you to update to a new ClockworkMod Recovery, it's version 5.something. However, the Bootstrap app in the marketplace is not updated past 220.127.116.11. Why is this a problem? Well, it appears that this new version of CWM makes the backups as data.ext3.tar and so on. 18.104.22.168 will ONLY work if the file is called data.img.
Now, I would have assumed that since using ROM Manager to make the backup pulled up this 5.x CWM that when using ROM Manager to restore the backup, it would do the same. But nope. Instead it resorted to the bootstrapped 22.214.171.124 and guess what happened. It wiped /system and then realized since there was no system.img, it couldn't restore anything, and left my phone unbootable. I ended up having to use the 2.3.20 SBF, then re-upgrade it to Gingerbread, install ROM Manager, and SPECIFICALLY BOOT INTO RECOVERY MODE FROM ROM MANAGER to ensure that it's the proper version.
Not sure if anyone has figured out a fix for this, but I thought I'd let you guys know so it doesn't happen to you. Another solution is to rename each of the files to *.img, however you also need to use a text editor (and one that preserves UNIX line-breaks) to change nandroid.md5 to also have the .img in the name, or it will say the sums don't match.
Hope this helps, and if anybody has figured out a way to update the bootstrapped version on the phone, I would love to hear about it.