After switching from a fullframe (film) SLR to the OMD for wildlife photos, I am not sure I would be willing to go smaller. The m4/3 format seems to give a very good compromise between performance and size-weight. I have been using the 75-300 M Zuiko lens for hiking/nature and birding with nice results.
For me the trade offs seem to be the need to stay at or under ISO 800, which means good light is a must, plus needing to focus the shots with S-AF-MF mode rather than C-AF. Moving the AF point is easy with the EM5 (just use the arrow buttons) and then if it latches onto a branch or leaf instead of the bird, you can quickly fine tune with the MF ring. Probably 90% AF and 10% MF fine tuning for me in these situations.
Th sensor noise level is noticeable when pixel peeking, it is a basic limitation of the smaller format, but the colors from the OMD are so wonderful that I realy don't see a problem – especially if you plan to spend money for fast glass and can keep the ISO close to base level.
If size were no limitation, a FF DSLR would be a nobrainer, but I don't know too many people who really want to lug around the extra lens and camera weight anymore.
I don't know what Olympus will announce, but the high grade glass investment vs. the cost of trading up the EM5 body in a year or two does not seem like a reason to wait if it does the job now.
I realized after posting this reply, that I had a pretty good example of the kind of shot you can get with the OMD and long telephoto lens.http://www.luminous-landscape.com/forum/index.php?topic=80472.0
This one is at the FF equivalent of 600mm in a compact and easy to carry size, and taken without a tripod. The DOF is possibly tight enough to make you assume it isn't from a smaller sensor camera.