I've compared gf1 vs ep2 for many many months before settling on ep2....and here's my take.
You can go gf1 if you want the faster af. However, be warned that even with the faster af on gf1, it's still a hit/miss affair doing sports photography or chasing kids, dogs, etc around. Try it out yourself, you will know what I mean. On thing to note though, gf1 has focus assist lamp which ep2 does not have.
M43 to me is a point and shoot replacement, hence i prefer one that can deliver the jpegs I want without having to take raw, meddle with photoshop, etc. If you like to fiddle around taking pics in raw and tweaking them, then gf1 may be ok because the shots are really pale (neutral) and doesn't look exciting to me at least. Also the jpeg right out of gf1 is crappy in comparison to ep2. The art filters on ep2 is really fun, you may argue you can get physical filters or do art filters in Photoshop.....then you probably belonged to the other group where pictures must be tweaked in order to feel satisfied or have fun....in that case, any crxxxy camera will do as it allows many hours of tweaking fun. The purpose here is to have fun taking photos not tweaking them (pns replacement remember??), getting physical filters also increases the costs.
Even without flash the ep2 does nicely....Usually, I take without flash as much as i can even when using pns. No built-in flash isn't an issue for me. You can easily snap on a flash on the hotshoe anyway.
With ep2, you can snap on a hi res evf which some says is nice while another group may find the evf difficult to use.
Point to note here, there is only 1 hot shoe, so you have only a choice of evf OR flash...can't mount both.
Video on gf1 uses avchd which gives you small file sized hi quality video. On the ep2 however, it is using avi and has a limit of 2gb file size. Video recording while zooming is pretty silent on gf1; on ep2, its loud and clear unless you buy a viddeo lens. Gf1 records in mono unless you get and external mic. Ep2 records in stereo. So if you do use video recording, do take note of this as it may be a breaking decision for you.
Finally comes the look and feel of the physical camera, the pen looks nicer and hence, a bonus. So only problem here is the oly lens. To solve that, simply get the panasonic lenses but beware the prices!!!
If you don't mind carrying a little more weight and a little bigger form factor, you can consider gh1 which gives you good lens, and camera body (good image and video quality). It is expensive though...but the 14-140mm lens will set you around $1200 if you buy separately, hence that makes the body in the $1000 range which is pretty reasonable as you get hd video, stereo recording, built in evf, better handling, etc.