With version 1.0 PSP you could use any old USB WIFI adapter(client) and trick your psp into thinking it was an access point(broadcasting ssid etc)
Well with 1.50 and up, you can't do that anymore.
I have discovered, that theres a USB WIFI adapater/access point in one for about the same price/cheaper!
It uses software to emulate an access point, so your little thumbdrive sized usb wifi adapter can be used as a true 'access point' for your psp, without having to buy a wireless router.
Router - computer - usb wifi ~ ~ ~ psp
The usb wifi gets the net from the computer(router) or if you have a usb adsl, that too, it doesnt matter.
btw theres a few of these on the market(different brands) so shop around, i was happy with this one so i gave the link.