DxO mark tests the GH1 sensor and compares it with the Canon 7D sensor!

“The GH1 even outperforms a hypothetical four-thirds sensor cut from a
(wider) Canon EOS 7D sensor (with the same pixel size). With its 50%
larger sensor surface (corresponding to 9 DxOMark Sensor points), one
would expect the 7D to have outperformed the GH1 by 9 points. Instead,
it achieves a DxOMark score of 64.9 — only slightly more than 1 point
above the GH1’s score of 63.6.

Except for the Olympus E410 and Olympus E510 (both released in early
2007), which have a DxOMark Sensor score of about 50, most four-thirds
sensors have a DxOMark Sensor score of about 55. The GH1 outperforms
them all with a DxOMark Sensor score of 63.6.

The GH1 outperforms other four-third sensors in terms of Low-Light ISO
(772 for the GH1 vs 571 for the Olympus E3, the second-best performer)
and Dynamic Range (11.6 for the GH1 vs 10.8 for the Panasonic DMC L10,
second-best performer).

Canon 7D’s performance is similar to the G1’s at ISO settings up to
800, and similar to the GH1’s at ISO settings from 3200. The GH1
outperforms the 7D for ISO settings up to 800.”

source: DxO mark