Map This!/Can I?...
Can I use GPS receiver X with Map This!? edit
Depends. If the GPS receiver supports NMEA and is self-powered, chances are that it will. However, you will also need a cable to connect the GPS receiver to the serial port of the PSP. GPS receivers that are not self-powered (except for the PSP-290) need more power than the PSP can supply, so these will not work.
Can I use Map This! in my car? edit
As mentioned in the Map This! splash screen: "Using this software while driving may be dangerous, distracting and illegal in certain areas. Please check your local laws and use caution!"
Can I use my TomTom maps and POIs in Map This!? edit
You cannot use TomTom maps or geodata in Map This!, because they are stored encrypted and because you probably aren't allowed to use this data anywhere else than on your TomTom.
However, TomTom POI files can be converted to Map This! format. Instructions to convert OV2 files to plain text can be found on the TomTom website. You can also use Global Map Download Tool to convert OV2 files.
Can I connect GPS receiver X to the USB port on the PSP? edit
So far, only the PSP-290 GPS receiver can be connected to the USB port. All other GPS receivers need to be connected to the serial port.