Looks like everybody and their grandmother's having this issue, and I can't update the iPad I'm using yet because of it. It looks like an error with doing a checksum check with apple's servers, somebody suggested downloading the IPSW file itself and making iTunes use that... but it's 700MB so I'm still downloading it...
http://forums.macrumors.com/showthread.php?t=1247406
http://forums.macrumors.com/showthread.php?t=1247406