Jeff Keller's Digital Camera Resource Page has a full review of the new Nikon D300 digital SLR, posted today. I've always liked Jeff's reviews because they are both reasonably comprehensive and posted as a single long web page, which I much prefer to the multi-page versions offered at other camera review sites.
The D300 is Nikon's hefty $1800 mid-high range DSLR, aimed at somewhat spendy photo enthusiasts, as well as professionals who don't need the full-frame $5000 D3 (or who'll use it as a secondary camera for the D3). It's a huge step up from the introductory D40/40x (and the D50 before that, which I own) and mid-range D80 (or its D70/70s predecessors), and replaces the very capable D200. It competes most directly with Canon's EOS 40D and Sony's A700.
Jeff gives the D300 a rave review, nudging it ahead of the 40D (which is admittedly less expensive). The biggest differences are in low-light and high-sensitivity performance, and in the apparently amazing new high-resolution rear screen, which also offers live view (something like what you get on point-and-shoot cameras) for the first time on a Nikon SLR.
I'm still happy with my D50, since my semi-amateur photography doesn't generally push its limits, but the D300 also shows what features you can expect to see trickle down to the low end in a few years. The pace of change in digital photography is remarkable—while quality lenses will still last, the era of keeping a single main camera body for decades, as many people (including me) did in the days of film, is over.