Unable to install 2.1 update

Are you using iTunes 8 ? Did you jailbrake your iPhone?

Uncheck the "automatically sync" box. Then quit and relaunch iTunes. Try the update again.

I had the same problem with the same error. Is your iPhone nearly full?
My iPhone only had 280 mb of free space left on it when I tried to update it. I went to the music tab and removed my music from the phone and tried it again. It's currently installing the update, so I'm guessing there wasn't enough free space on the phone to fit the update.
If your phone is nearly full like mine was, I'd set it to not sync something, just to give it a little wiggle room. After you've installed the update, then you can probably sync it up just like before.

I have the same problem when trying to update to 2.1 except that my error contains the number (-30) instead of 21 that you have.

This morning i had Error (19) I restarted my iBook and Iphone. Also when I synced my iphone into itunes again I made sure it wasn't trying to automatically sync itself. Now i'm just waiting for the backup to finish lol...

First let me apologize for just posting in middle of replies.. I couldnt find the post button(?)..

So just got home and was updating phone, put it in airplane mode before updating...And I got this message..please someone help me..

"The iPhone cannot be used with iTunes because the information required for activation could not be
obtained from the iPhone"

Also, Yes I did update to iTunes 8...