Face it: Sony's backed you into a corner. You can't play PS3 online without downloading the 3.21 firmware update, but if you download, your precious 'Other OS' is forfeit -- and there may be side effects. You could wait it out, hoping Geohot comes through with a workaround. You could even sue Sony for a refund if you live in Europe. Or, if you're not terribly worried about the security of your PSN account and personal information, you could instead input just ten digits into your PS3 and magically bypass the entire issue. Though technically minded individuals have been skirting Sony's firmware checks for years now using private proxies, homebrew developer Aaron Lindsay decided to set up a public server at 184.108.40.206, which allows anyone to piggyback on his success in mere seconds -- we tried it ourselves, and it worked like a charm. By doing so, you run the risk of giving Aaron all your credentials... but if the odd identity theft doesn't break your stride, you can jump right back into that Uncharted 2session by entering the above number into Internet Connection Settings > DNS Setting > Primary DNS.
Update: As of April 13, the proxy DNS trick no longer works. We're not sure why, but considering how long the hacking community holds a grudge, we expect a workaround in due time.