This actually happened to me as well with my iPod touch when I moved from a Dell laptop to my Macbook Pro. It was working fine for a while, then one day I started having problems with the touch connecting to our home network. Troubleshooting said do a restore on it and most of the time that fixes the issues. Well, I did the restore, and the networking started working again, but I lost my $20 January software upgrade. Don't know why, but it wasn't in the backup when I moved from the windows machine to the mac. I'd been syncing it just fine, and it had been backing up, but it just didn't back up that package. I went into the iTunes store, to the previous purchases and found it, but there is no longer a link to download it again. I reported a problem, and basically said I was SOL as the software is no longer available. I was told, however, that the 2.0 software upgrade ($10) includes the stuff in the January software upgrade. I was going to get the 2.0 software anyway on the touch so it would be more attractive to sell once I have my iPhone (won't really need the touch anymore), so that satisfied me at least for now.
But unless you have that software application backed up within your iTunes, you're out of luck until 2.0 comes out - which should be Thursday anyway.
