Here's a problem:
I listen to my iPod on the way into work and at the gym.
I listen to Last.fm at work.
The iTunes that is connected to my iPod is the one at home, where all my music is.
Last.fm will not accept submissions earlier than the last submitted song.
So, what happens is that I listen at the gym, listen at work, but when I sync my iPod at home, none of my gym songs, which occurred before my work listening, get added to Last.fm.
But, I can't link the iPod to two computers and sync when I get into work and get my recently played data into the system before I start listening at work.