I saw a review and they said that the focus is slow in the lens-camera combination of a 12-60 and a E-520.
I think it's still fast, but it's just that it works even faster in the E3.
Gordon reviewed the 12-60 http://www.cameralabs.com/reviews/Olympus_Zuiko_Digital_12-60mm_SWD/
The only compatibility issue was that since the lens is relatively large, the e520 popup flash doesn't raise high enough, and you can get some barrel shadow using wide-angle + built-in flash. A hotshoe/external flash may be better if you're using flash photography
Panasonic G3: 9-18mm, 14mm, 20mm, 45mm