Just tested it on my iPhone, it was very easy for me to go down from 3.1.3 to 1.1.4, just DFU & Restore with iTunes 7.5 on freshly installed XP. It was done the first time, no errors (just the baseband error but that's normal, solved with iBrickr 0.91).
But i see that is not so simply to restore iOS 2.x, i don't know why. It's possible, because i have have now 2.2.1 on my phone, but it's a little trickier than 1.1.4.
To install 2.2.1, i downgraded to 1.1.4 and than shift-restored a custom firmware (created with PwnageTool 2.2.5) with iTunes 8.0.2. and the iPhone powered on, no DFU or Recovery. No errors, just fine.
I tried restoring 2.2.1 in DFU but iTunes gave me the "iPod software update server could not be contacted" error, while it worked with iPhone powered on normally. Apple logic.
For these tests, i suggest this method:
create a fresh, clean XP install in VMWare (or other VM platform). Than install just the drivers & VMWare tools. Reboot and than take a complete system snapshot
BEFORE installing any other software on it.
So, if you have a problem with iTunes (and i'm sure you'll have

), just restore the snapshot in a few seconds and you're ready again.
With this trick, it's possible to test different iTunes versions without the hassle of removing it every time (iTunes for Windows is kind a crappy software, is difficult to remove completely)
PS. Sorry for my english, i'm from Italy & still learning