PSP firmware ver3.70 released in Japan
But you may want to wait for the European version

Sony Japan has released the latest firmware for PSP owners to upgrade to. Version 3.70 doesn't offer anything that will stop hearts through sheer excitement, but the additions are nevertheless bound to be welcome by many.
First on the list, then, is custom themes, enabling you to set these on your PSP. Sony promises several themes based on its more popular titles to become available for download through the official PlayStation and PSP sites.
Okay, it's possibly not something that was top of your wish list. How about the ability to look at photos and listen to music simultaneously? That's not bad, right?
If that doesn't impress you, perhaps the functionality of assigning buttons in Remote Play will – there could be some significant uses for this, after all.
Lastly, the remaining new features focus on movie playback: scene search support has been added, together with sequential playback for those who like to chain their video footage together.
As mentioned, this is the Japanese firmware update. Theoretically, it should work on any PSP but it might be worth waiting for the official European which should be here any day (typically, there's never more than a week's difference between the territorial firmware releases).
And you may want to wait for this and future official European firmware updates because one of them will give access to the Go!Messenger application – a function that, currently, will only be available to UK PSP owners.
Ver3.70 was actually the version expected to provide said application, but with the service not due until January 2008 it seems more likely this will form part of a subsequent firmware update.