theres another possibility - the wpa options of some routers can differ from those used in the psp (because the psp is updated all the time - i have this very issue and can't use wpa-psk)
now i don't fully understand the details of why it doesn't work but maybe this is his problem. to check this make your connection without encryption (at access point and psp) and see if its fine. if its still not working it could be a firewall issue. if it worked fine before and only the psp has changed then it really has to be something on the psp. so then you investigate the methods used to downgrade (because the keys issue can also cause effects like this).
good luck.