I've only been riding for a short while, and I decided to get a road bike when I started doing longer rides -- the idea of doing the MS-150 on a mountain bike scared me. I'm nowhere near in enough shape to pull that off (although, i did get passed by people on mountain bikes when i was on a road bike!)

My LBS sells mostly Trek, so I started off trying the Trek 1000. I'm short with short legs, so I'm a little bit limited as to bike frame size (43cm, 650c wheels). I took it in for a few adjustments after a few rides, and I ended up trading it in for a Pilot 2.1 (which is similar to the 2100). It's supposed to have a more "natural" riding position, but it's still a "road bike" (I think). It has road bike gears and skinny tires. Maybe you could take a look at the Pilot line...

And with the 2008 bikes due to come out soon, you could probably get pretty good deals on the 2007 models. I got mine for right around $1000.