I currently have an Olympus E510, I'm getting to the stage where I'm out growing the camera and would like to upgrade to something newer.
I was looking at getting the Nikon D90 as the reviews and spec's of the camera are great. I'm now stuck with a little dilemma. Nikon just released the spec's for the D3100, which on paper is better than the D90. So do I...
Get a D3100 because the spec's are better
Go for the D90 because spec's aren't everything
Wait for an an announcement of the D90's replacement?
Interestingly enough, I started out with an E-510 an now have a D90 so I know what you go through trying to decide!
Although the D3100 has some rather interesting headline news it's still a very different camera to the D90. Theres the build quality, added features and even the top plate LCD - you get very used to having one again and I wouldn't loose it for the world. Also, the D90 (and presumably it's successor) has a pentaprism viewfinder (rather than the cheaper pentamirror found on all lower Nikon models) which is worth the extra money alone.
One other important thing to check is whether you definitely want to change systems. I don't know how much Olympus kit you have but you either have to keep it or sell it. I sold mine but I got lucky and sold most of it to a friend as a basic package and the rest on eBay.
Otherwise, it's rumoured that the replacement for the D90 will be released at Photokina which is next month. It's rumoured to have a 16MP sensor, the noise reduction alogrithms of the D700, a part aluminium body, 8 fps and the same video as the D3100 (1080p and full video AF).
It these specs are anywhere near right (and they make sense given the current direction from Nikon) I'd wait for the pre-Photokina announcements and see if they release it there.
Finally, I will add that I love my D90. It's a real step up from the lower range Nikon models, and the E-510 as well. It feels more like a serious camera and it behaves like it as well. In short, I've nothing but praise for it and, if it's replacement comes out soon, the D90 will be getting cheaper - you might be able to pick up a bargain.