Homebrew developer Art has come out with version 1.0 oh his WiFi Sniffer
Road Dog for homebrew enabled PSP users.

This nifty program runs like any other homebrew application but instead of
playing a game, it continuously searches for Wifi hotspots as you
approach them. If you haven't quite got the grasp of this program yet, think of
it as an internet finder for your PSP or other mobile devices such as a laptop
or anything else that uses a wireless internet connection. You can the airwaves
with Road Dog, and the program displays all currently detected
wireless connections within your PSP's reach.

You can essentially pair this with a PSP car charger and have a fully mobile
Wifi hotspot finder! This program is a MUST HAVE if you are on the go a
lot and use your PSP to browse the web or just want to use your PSP to find all
the wifi hotspots where ever you go.