The new Nikon D5000 is, in essence, a D90 with the top LCD and focus motor removed, an inferior LCD on a tilt-swivel mount, a lousy viewfinder, only one control dial, and what appears to be some intentional crippling of its continuous shooting rate (so as not to further embarrass the D90). While it can “only” manage 4fps, it can keep it up for much longer than the D90 (11/63 vs. 7/25 RAW/JPEG frames). Personally, given the choice, I’d pick 4fps and 63 frames over 4.5fps and 25 frames.
On the positive side, the D5000 offers the D90’s focus system, fast shooting, excellent image quality, and some nice features that previously haven’t made it down to consumer DSLRs like switchable grid lines. The down side is that Nikon still doesn’t offer 14-bit RAW until you get to the D200/D300. The tilt/swivel LCD seems like a useless gimmick — especially considering Nikon’s less than stellar Live View mode (but I guess it makes the D5000 more like a camcorder).
So, my take, the D5000 replaces the D80 at the cost of an inferior viewfinder — a serious bummer — but doesn’t really address the new Canon EOS T1i or the Panasonic GH-1.