I'm a champion of the Trek 1.5.
If I were going for a 2.X Series, I'd go with the 2.3.
Why? Both the 1.5 and the 2.3 get great reviews from bike mags (the 1.5 won Best Entry Level Road Bike by Bicycling in '09 and is currently an editor's choice for '10).
As for the 1.X vs 2.X Series discussion...that will depend on you really.
I went with the lowest series because I always knew I'd upgrade to a $3K bike eventually.
I don't personally buy by components groups on bikes I'm going to upgrade anyway.
If you know enough now to know that this will be a very long term bike,
then I'd definitely say go for the 2.X bike. It's an excellent bike (as is the 1.5).
But in my humble opinion I'd go with the 1.5 or the 2.3.
Either bike will not need upgrading for a long while in their respective categories.
(1.5, entry level bike, 2.3, recreational rider/possible entry level racer).
The only real difference in the two from what I've come to learn is (besides components groups) are the carbon stays on the 2.X bikes which give it a slightly better ride that really becomes noticeable on longer rides.
Personally, I ended up falling in love with my 1.5 and am putting about $2,000. into it.
Wheels, saddle, cockpit, groupset...for that money I could get a new Madone, but it wouldn't be my upgraded 1.5.
1.X Series bikes are by far the best alumnimum frame bikes you can get for the money.
The only thing that beats it is a CAAD9, but now you're talking about a bike designed more for racing (which is also on my hit list).
If you're just venturing into road biking and want to get a feel for it more, the 1.5 is a great choice to get your feet wet with. I have guys with $10,000. bikes that like my bike and compliment it even on how it rides.
If you're more seasoned, the 2.3 is a great bike too.
The jump up in price between the 1.2/1.5 and the 2.1/2.3 is really not much when you figure how much MORE you get for the relatively small difference.
If I had to do it all over again knowing what I know now however, I'd still get the 1.5.
Good luck and let us know what you choose.