The 4.45 Update has been pulled. 4.41 is now considered "Up to date".
I've been watching the ****storm unfold on the forums and I've seen the affected posters layout as such:
Aftermaket HDDs, 500GB-1TB ~90%
Stock 500GB models ~8%
Stock 120GB models ~2%
Stock 1st-gen fat models 0
The 2% may well be people complaining that do not actually have a broken system. People lie for no good reason sometimes, and the PS forums have never struck me as a deep bunch.
There's about 3 new posts every minute for the last hour, and Sony's still apparently trying to get some people to pay for a repair.
If you're affected, my advice would be to simply wait. This will be an all-hands-on-deck sort of hotfix, so don't do anything drastic. Find a convenient flash drive and format it and wait for the hotfix which will be delivered via that flash drive. Many people have turned a bad situation (hung on XMB) to a data-loss situation by trying to use the recovery mode. Just wait it out.
EDIT 2: A commonly reported workaround if you've upgraded your HDD and it's currently unusable is go dig out the original HDD, or a comparable HDD below 500GB, and use that in the meantime. Obviously no saves unless you have PS+ cloud saving, but it'll get you going in the meantime.
EDIT 3: If you're still seeing an update prompt, reboot your system and sign back into PSN, it should go away.