I found a way to restore to 2.1 from 1.1.5
Download the 2.1FW and go into your itunes restore folder, I think it's in c/documents and settings/application/apple/itunes restore or something, I can give a direct and proper folder when I get home from school but if you can navigate into this folder you should find your 1.1.5 retore.
Take 1.1.5 out and place onto your desktop, place 2.1fw into the restore folder and then open itunes or restart it and click restore.
This "did" work for me and then i winpwn'd it and jailbroke my itouch 2.1
When you winpwn it just navigate to your itunes restore folder and click on the 2.1 firmware.
This was the only way I found I could update to 2.1 because shift +clicking + opening fW did not seem to work so I winging it.
To get back to 1.1.5 just replace your 2.1 with 1.1.5 and place 2.1 on your desktop or delete it if you like.
I did this when 2.1 came out and winpwn 2.1 itouch jailbreak was just experimental.