gdgt

It of course depends on what you want to shoot. With the 2 lenses you mentioned, it sounds like wide and short tele (a 50mm on a APS-C is about the same as a 75mm lens on a full frame camera).

I'd say these are the "must have" lenses for Micro Four Thirds (they are also what I have; so sue me ;) ):
Wide Angle: Olympus 9-18. The Panasonic 7-14 is a better lens, but it's a lot more expensive and a lot bigger and heavier. The 9-18 gives good contrast, still offers a 90 degree field of view at the wide end, and is very small and light. If you're okay with a prime for your wide angle needs, the Olympus 12mm is hard to beat. It's not cheap, and the field of view is equivalent to a 24mm full frame lens , but it knocks it out of the park optically.

Normal: Panasonic 20mm 1.7. It's fairly inexpensive now, renders beautifully, has good contrast, quite fast, and is a pancake. The only downside is that it's a bit slow to focus. The Panasonic 25mm is a quite a bit better lens, but you're paying about 2x as much for it. If you like the field of view better, it may be worth it, but if you're fine with the field of view from the 20mm, I'd go with that instead. I'm not a fan of the Olympus 17 2.8. It's not that bright, it's not that sharp, and not that high in contrast.

Tele/Portrait: Olympus 45mm 1.8. This is such a great lens. Yes, you can get a Canon or Nikon 50mm 1.8 for much less than this one, but this is a fantastic portrait and short tele lens with great contrast and detail. It's really tiny as well. If you do a lot of macro work, you should probably consider the Panasonic 45mm 2.8. It's not nearly as bright as the Olympus, but it's the only macro lens for Micro Four Thirds (for now).

This whole kit - E-P3, kit zoom, 20mm, 9-18, and 45mm - will all fit in a small camera bag, i.e., one designed for JUST a DSLR with kit zoom on it. Pretty cool, huh?

Those are the must haves. If you want a super zoom to replace your kit zoom, I'd go for the Olympus 14-150 is a solid daylight lens. It's about as good as the kit lens optically, and doesn't do well in low light, but that's going to be the case with any super zoom, really. If you're going with a Panasonic body, I'd go for the Panasonic 14-140 OIS instead. Olympus has in-body stabilization, while Panasonic puts it in the lens. This means that any lens is stabilized on an Olympus body, but you need a in-lens stabilization for a Panasonic body. Most Panasonic lenses are optically stabilized, other than the 7-14, 20mm, and maybe one or two others.

The Olympus 40-150 is a better lens and can often be bought for around $200-250, but the problem is you'd be carrying 2 lenses instead of one. If you're okay with that, go for the 40-150 (or the Panasonic 40-200 OIS, if you're using a Panasonic body).

I really want a micro 4/3rds camera, but I already have a dSLR, so it's hard for me to justify buying one. But if I was in the market for one, I'd get the Panasonic LUMIX G 20mm f/1.7 (gdgt.com­/panasonic­/lumix­-g­/20mm­/f17­-asph/). It's really fast and I like the contraint of prime lenses.