I'd stick with Nikon.
If you're just going after a good everyday lens, the 18-70 mm lens that comes with the D70s kit is supposed to be very good.
If you get a Canon, try getting a 20D. The 350D is much smaller and lighter than what you're likely used to. Sounds great, but I don't find this necessarily attractive in a camera. However, some people do, so you'll need to go to a store and hold all the cameras first.
The Pentax K100D is a lot of bang for your buck. I'm sure they have great lenses, and Pentax camera bodies are supposed to be fantastic, but since they're not as big in digital photography as Canon or Nikon, it's going to be a bit harder to find Pentax lenses. However, you can always get 3rd party lenses from Sigma, Tamron, and Tokina. They make lenses with the K-mount as well, so it's usually easy to find lenses for your Pentax from these companies. Some of the lenses they offer are just as good, slightly worse, or even slightly better than what Canon and Nikon has to offer (they are usually a tad slower at focusing, though
).
Oh, and while you can use older lenses on digital bodies, I don't think they're supposed to be as good for digital because some of the light reflects off the sensor, then reflects off the inside face of the lens glass, and then back towards the sensor, which means you'll likely get lower contrast or something. Wait, is it contrast that goes down because of stuff like that? My Medical Imaging professor would kill me if he found out I couldn't figure it out.