PSP possible downgrade with UMD 1.5 disk
This is something I read over at PSP Spot and it has been confirmed as working, however, try at your own risk.
What you need:
UMD European 2.0 disk
PSP 1.51 or 1.52 version
Any UMD with a 1.5 update on it (Midnight Club was used so stick with that if you can.)
Apparently what you do is you begin the update process using the 2.0 disk that was bundled with the European PSP's and wait until it gets to the part where it asks you to plug the AC Adaptor into the wall. At this point you take the UMD disk out and then insert the version 1.5 game. By this time the PSP has already passed the version check and continues to load the software (in this case now being 1.5). I have no idea if this is true or not as I haven't tried it (my PSP is already version 1.5). Go to the PSP-Spot forums for more info. (For some reason it will not let me input the URL.)