03-09-2009, 10:22 PM
A very highly regarded and relatively lightweight lens available for Nikon is the Tokina 80-400mm f/4.5-5.6 ATX 840. You can hand-hold it if you keep the shutter speed up (it's not stabilized and neither is the D90 body) but it does have a tripod mount. And compared to other longish lenses it's relatively inexpensive: $650 at B&H. 400mm will get you into some nice birding territory, not to mention the rugrats & yard-apes.
http://www.bhphotovideo.com/c/product/42..._Lens.html
Edit: Also you might want to look at the Tamron 18-250mm f/3.5-6.3 Di II Macro. It's reviewed here:
http://www.photozone.de/nikon--nikkor-ap...rt--review
In it's Pentax-badged version, I know that people are generally very happy with it.
http://www.bhphotovideo.com/c/product/42..._Lens.html
Edit: Also you might want to look at the Tamron 18-250mm f/3.5-6.3 Di II Macro. It's reviewed here:
http://www.photozone.de/nikon--nikkor-ap...rt--review
In it's Pentax-badged version, I know that people are generally very happy with it.