Interest in using the Panasonic G1 with adapters to mount a variety of older lenses appears to be high. Stephen Gandy at Cameraquest quickly sold out his first batch of adapters, apparently just slightly in advance of their actual arrival; he has another batch coming on March 1st and has promised that a page on CameraQuest about them will go up tomorrow evening (I assume you will find it under Classic Camera Profiles —> Lens Adapters). Meanwhile you can get the Micro 4/3 to Leica M adapter from Japan Exposures. Remember these are not mass-market products, so the prices aren't low.
I'm pleased to say that a Panasonic G1 should be arriving at TOP in the next week, to be subjected to our typical putterings and natterings a.k.a. "review." (I also finally bought a new camera for myself, but I'm not going to tell you what it is until I've got some actual work done with it under my belt. That will probably be sometime during the Summer.)
These two nifty pictures come from a page on the Japanese site DC Watch. I can't read the page, but I think you'll agree the pictures are eloquent!
(Thanks to Vlatko and Dirk)
ADDENDUM: Coincidentally, DxOmark.com has just published its results on the G1 sensor.