Another vote for getting a "road bike" -- but get one that's capable of handling rough roads over long distances and all kinds of weather conditions, not a twitchy little race bike with 700x18 tires that can't handle fenders. The suggestion someone made of a cross bike seems like it would work well. Or the suggestion to go used -- my commute bike is an '84 Trek 400. Any of the Trek 400's built between '84 and about '90 would work well as a commuter bike, or a long-distance riding bike. I particularly like the '86 and '87 model year since it was built around Reynolds 531, which has a very lively feel. All can take 700x30 tires with fenders. These Trek's sell on EBay for typically between $70 and $200, depending on condition and equipment. There are several other Trek models that would work well, but the numbers change all around and some might be on the racier side and harder to run with fenders.
"crisi13" is right: If you get a hybrid and then decide you really like bicycling -- which is the most likely outcome from riding twenty miles a day -- then you'll be wanting to get a road bike.
Oh, and don't let anyone tell you that a twenty mile a day commute is nothing. I've commuted regularly for twenty years. I've ridden my current twenty mile a day commute _every_ day that I've gone in to the office, in all sorts of weather conditions, for 3-1/2 years running. It's a thousand feet of climbing a day. Temperatures between 10F and 105F. Rain, sleet, snow. The workout is as good as you care to make it. You can ride at maximum possible effort all the way. Or you can take it easy to recover from the 600K you rode the previous couple of days. You'd think that you'd get so used to riding your 20 mile commute that you'd never notice it, but for me there are still days when I get home feeling just thrashed. Frankly, I don't know anyone in this area who has a much longer commute who rides every day, because it ends up being too far.
Nick
PS, In case anyone wants to say that I only think 20 miles a day is a decent commuting distance because I'm a wimp: Mileage totals since I started bothering to keep track ...
2004 3724
2005 7021
2006 8317
2007 9614
2008 2600 (so far)
So far this year: three centuries, five 200K's, a 300K, a 400K, and a fleche.
Just under 40000 miles total since I bought my "new bike" in 1990.