A French website called xabox.com claims to have posted pictures of the Xbox 360 motherboard.
If this is really from the Xbox 360 then it must be taken taken from the recently released Xbox 360 beta developer kit which, according to rumors, includes a test version of the Xbox 360 GPU from AT but still the dual-core PowerPC 970 processor from IBM. Here's the leaked picture:
TeamXbox writes the two huge fans are probably covering the Xbox 360 processor (right) and the ATI Xbox 360 GPU (left).
More details and photos at TeamXbox