In a rather interesting turn of events, it appears that PS3 users with custom firmware are able to log back into PSN - without upgrading to 3.61, the firmware released by Sony as part of its efforts to secure the network. Currently in terms of custom firmware, users can only install modified versions of version 3.55 or below. Versions 3.6 and 3.61 have been secured by Sony in such a way that they have yet to be "hacked" in any way that allows homebrew or piracy. Theoretically, this should prevent anyone from getting on PSN or running the latest games without upgrading.
Without giving too many details away, a modified version of 3.55 was released that made a retail PS3 act like a "test" PS3, or a debug PS3. Normally, this wouldn't allow you to get onto the real PSN, the console would attempt to access one of the developer networks that Sony uses instead. Still, where there is a will there is a way and some clever users found a way to trick their retail-cum-developer consoles to play games on the actual servers that regular PS3 people use, effectively letting them back on PSN.
That was a few months ago and about a week after this was unveiled, PSN was taken down for its 3 week hiatus. At first, people speculated that these custom firmware users were the "intrusion" that Sony spoke of, before Sony made us aware of the larger attack that took place.
Most people expected that when Sony brought the PSN back online, these users would be locked out as a matter of security and, to a point, they were correct. Everyone was forced to update their console to 3.61 before getting online, however just a few short days after the network came back, people claim to have already found a way to "spoof" their consoles without updating. While the information originated from a Brazilian website, this information is already spreading amongst various other sites with several users confirming that it does indeed work.
If this is true, and it's a big if, it does bring into question just how much more secure the new PSN really is. Hopefully this is just a small glitch on Sony's part and will be reconciled soon.