DxO's "DxO Mark" methodology and eponymous website are together an attempt to compare digital camera sensors at the chip level, based on true apples-to-apples measurements from RAW files before all the in-camera image processing has taken place. They've built up a good-sized database of tested cameras now, with a number of models from pretty much every manufacturer. Their most recent results show the Panasonic DMC-GH1's sensor to be an unusually good performer, actually edging out those from such vaunted SLRs as the EOS-7D, and clearly standing out against the rest of its Four Thirds brethren.
The larger size of the 7D's chip lets it edge out the GH1's overall, but only just, and on a per-mm basis, the GH1's sensor comes out ahead.
IR takes no official position relative to DxO's DxO Mark test methodology, but it's an interesting approach, and we generally applaud the attempt to dig beneath cameras' JPEG processing, to get at the underlying image quality factors at the sensor level.