I've had an Olympus E-510 since July and really love it. I wish I could upgrade to the E-3/12-60 but can't justify the cost for what is just a really enthusiastic hobby right now.
I know much more about the E-3 than the D300.. I've only handled the D300 briefly. My random thoughts (trying not to be too slanted to Oly because the D300 truly rocks, but Oly is what I know about), in no particular order:
-The D300 screen is gorgeous.
-The fully articulating E-3 screen is a perfect match to live view.
-The AF speed of the 12-60 on the E-3 is unbelievably fast.
-Both have sophisticated focus systems, though I'm sure with 51 vs 11 points the D300 has the advantage here in intense shooting situations. I really like how the active points are selected on the E-3, and the 3D tracking on the D300 sounds very cool for sports shooting and the like.
-The D300 has Nikon's new dust reduction system which may be just as good (not sure), but I know first hand that dust is a non-issue with Olympus. I'm not careful at all when I change lenses on my 510 and the only thing I've had to use the blower I bought for is to get dust off the focus screen and mirror.
-The E-3 viewfinder is large and bright, a real leap forward from any previous 4/3 offering. Both have 100% VF coverage, which is nice.
-You'll definitely have more prime lenses to choose from with Nikon. On the flip side, the primes that are available for 4/3 benefit from the in-body IS. I've really liked having my 30mm f/1.4 be stabilized.
-Physics dictates that a larger sensor will have lower noise given similar sensor technology.. no way around that, but the difference isn't so large now as it used to be. Here's a link from someone that shot the same swimming event at 1600 ISO on both the E-3 and D300:
http://forums.dpreview.com/forums/readflat.asp?forum=1022&thread=25765206
-All Oly lenses above the "standard" grade ones are weather sealed, which complements the weather sealed E-3.
-If you get the D300, do it justice and get a better lens than the 18-200.
