A post from the Innerbits Blog confirms that we won't be seeing a game-wide XMB update to allow custom music soundtracks, because the feature is already there, it's just up to the developers to use the plug-in in their games, so we can safely assume that most games from now on will have the feature.

Other features worthy of note are integrated PSP functionality (for use as a display) and a screengrab option (at only 3MB), both of which are available now for developers. The memory footprint of the XMB itself has been reduced by a total of 32 MB since the 1.60 update, which is great news for devs as they now have more memory to play with.