Olympus PEN E-P1 vs Panasonic Lumix DMC-GF1
Size comparison from leaked photos - the GF1 and E-P1 look to be roughly the same size, although the bigger top mode control button on the GF1 makes it taller.
(see a larger version)
Panasonic GF1 Advantages
Internal Flash - A big "pro" in the GF1's favor is the availability of a pop-up flash. A nice, built-in flash is always a welcome convenience factor, and I think Olympus made a mistake in deciding not to include one on their E-P1.