Sony's Eric Lempel has posted a somewhat exhaustive Q&A session on the PlayStation Blog, which details everything you may or may not have wanted to know about the console's major, upcoming firmware update. While I'm sure it was meant to be helpful, a read down his list of responses does little but drive home the point that, as neat the implementation of the trophy system is, the addition of the in-game XMB is nowhere near as useful as most of you thought/hoped it would be.
As an example: does the in-game XMB allow voice-chat? No. Can you browse the internet while playing a game? No. Can you access the in-game XMB while playing a PS2 game or watching a DVD/Blu-Ray? No. Can you change any visual settings while playing a game? No (sound settings, however, gets a "yes"). Can you access the PlayStation Store while playing a game? No. Does it let you invite a friend to a game who's playing another game? No. See where this is going?
If the in-game XMB forces you to quit the game to perform most of the functions you'd want to perform in it...then there's not much point having it available in-game, is it? Course, Sony probably knew this, which is why it's only been implemented now - after you all cried yourself hoarse over it - and not at launch.
Firmware v2.40 FAQ [PlayStation.Blog]