This is our standard studio scene comparison shot taken from exactly the same tripod position. Lighting: daylight simulation, >98% CRI. Crops are 100%. Ambient temperature was approximately 22°C (~72°F).
Olympus E-P1 vs. Nikon D5000
Olympus E-P1: Olympus 50 mm F2 Macro lens, Aperture Priority, ISO 100
JPEG Large/SHQ, Manual WB, Default Parameters (Standard), Self-Timer
Nikon D5000: Nikkor 50 mm F1.4G lens, Aperture Priority, ISO 200
JPEG Large/Fine, Manual WB, Default Parameters, Self-Timer
7.4 MB JPEG (4032 x 3024)
4.6 MB JPEG (4288 x 2848)
It's clear to see that Olympus - as usual - is doing a great job with its out of camera JPEGs, and at a pixel level the E-P1 produces, sharper, crisper output than the Nikon, and the punchier default contrast, sharpening and saturation mean that the results are more immediately appealing. You're not going to see a huge difference at normal magnifications (and turning up the Nikon's rather conservative sharpening evens things out a little), but there's no denying that the little Olympus is giving the D5000 a good run for its money (though again the rather harsh highlight roll-off produces slightly clipped tones at ISO 100).