So Apple released iOS5 yesterday, and Apple servers are getting slammed with people trying to download iTunes 10.5 and iOS5.
I downloaded and installed iTunes 10.5, plugged in the iPad, and clicked Check For Updates. "You're on 4.2. You're current!" - it says, or something like that. You would think clicking Check For Update over 100 times would have burned the message into my brain.
I even downloaded the iOS5 IPSW (link below) and tried to Shift-Click Check For Updates, but I got the pesky 3002 error.
Evidently, you can't upgrade your iPad to iOS5 from iOS4.2. It's one of those "you can't get there from here" deals.
You CAN, however, upgrade from 4.2 to 4.3.5, THEN to 5.0. Yay! Hoops!
Download (link below) and upgrade to iOS4.3.5. Once that's done, iTunes will prompt you to upgrade to iOS5.
iOS4.3.5 for iPad1
iOS5 for iPad1
Hope this helps.