In summary
* The E-P1 produces better JPEGs with brighter (yet more realistic) colors, punchier contrast and cleaner pixel-level detail.
* Although the E-P1 has marginally more highlight dynamic range (at ISO 200) than the GF1, its metering tends to skate dangerously close to over exposure, so - without manual intervention - you get more blown highlights with the Olympus. The GF1's metering is definitely more reliable.
* The GF1's focus is faster and a touch more reliable than the E-P1 (which occasionally gets it completely wrong).
* Despite being nominally longer (45mm vs 42mm), at short focus distances the Panasonic kit zoom is visibly wider at the tele end than the Olympus.
* Likewise, at the wide end the Olympus is slightly wider (we suspect this is down to Panasonic's stronger distortion correction, at least in part).
* In our lens tests both Panasonic kit lenses is measurably better than the Olympus equivalents, and the in-camera removal of CA is a bonus. How much of this you seen in most shots is debatable.
* Although covered elsewhere it's worth repeating here that the E-P1 does slightly better at very high ISO settings (1600+) than the GF1.