I would like to hear from anyone who tried this method specifically. It has survived several shutdowns so far. UPDATE 3 (June 17, 2008): As a last-ditch effort, I tried turning it off and removing components (battery and DVD-drive), and after removing the DVD-drive and putting it back in, the wireless network adapter is back. I will need to send my notebook back for repair. UPDATE 2: None of the suggested fixes worked. I'm about to try a solution suggested by one of the users.
I found a forum thread about the same issue with other HP models here. UPDATE 1: The BIOS update did not fix the problem after all. NOTE: This BIOS update seems to be specific to the tx1220us. You might also be interested in the other software updates. I just updated to the latest version (F.20), and it seems to have fixed the issue.
I was about to send my tablet back to HP for repair, and I'm glad I waited a while. I think the problem was introduced in one of the later BIOS updates (F.1X). The Device Manager didn't show any wireless network card hardware at all, and the problem comes and goes with each shutdown. My HP Tablet PC (tx1220us) had the problem of not recognizing the wireless adapter hardware for a couple of weeks so I've been unable to connect to the Internet wirelessly.