When I sign into PSN, the logo spins briefly, sound cuts out, the logo freezes, and then a few seconds later, the system hard powers off (shuts down). The lights don't change; no red flashing, etc. Just suddenly it's off with no warning. It happens 100% of the time. If I don't sign into PSN, everything seems fine. No crashes.
I thought I had a defective unit but I got a replacement at a Sony Store and still have this problem, so I suspect it's something with my account or PSN itself or something. My account is in good standing, worked fine with my PS3, etc etc.
If you use bind 9 as your DNS server (which some routers do), you can fix it by adding "minimal-responses: yes;" to the global "options" section of your configuration and restarting bind. Once done, you should be good to go.
The real fix is on Sony's side in their networking code, but the above is a workaround.