Since there is no official 1.7 release for the PSP, I was wondering what the difference between the current 1.7 stable and an official relase is? Can anyone shed some light on the differences?

   

I assume you mean between the PSP daily build from Buildbot and a release build... Well, mainly that the daily build is automatically generated from the latest source code for development branch (master) and release candidate / last release branch (stable). As the notes here indicate:
http://www.scummvm.org/downloads/#daily

This means it likely has had no testing and could be broken in various ways and/or crash... It is also likely compiled with debug options so it may run slower than a release build and/or produce debug output.