Browsing through the P2 forum I’ve discovered a mention of a fantastic child theme for P2 by Jonathan Warren.
Child themes are notoriously tricky to get to work with the P2 parent theme, so you’ll have to do a bit of fiddling with some core files – but the results are great.
Check out a demo of P2 Responsive here, or download the Child Theme from GItHub (comes with instructions on how to make it work).
You may also be interested in Jonathan’s release post about his work.
P2 User Guide 
Thanks for the link, I’ve just updated the theme so it no longer requires modifying the parent p2 theme. You can grab the latest version from GitHub.
Excellent – loving your work, Jonathan!
A much needed addition! There are now a couple of responsive P2 children in the WP theme repo, too: Mercury (http://wordpress.org/themes/mercury), which I think looks great, and Houston (http://wordpress.org/themes/houston) which looks less promising but has a couple nice touches. I haven’t played with either yet beyond installing and doing a live preview.
I also noticed P3 and RoundTable, a couple of non-responsive P2 children not listed here.