Kawasaki Ninja 250r
This is considered the be all end all of beginner bikes. It's reliable, not over powerful, cheap even brand new ($3400), and the new 2008 model looks sexy. It is a great bike to learn on, and even being a 250cc, it will still go 0-60 in 5 seconds, which is faster than all but maybe 5% of the cars on the road. It's not a great bike for doing lots of highway mileage, but it does get good gas mileage and dirt cheap insurance. It's also very light. If you're looking to just get a learner's bike to sell after a year or two, this is your best bet, as resale for these things is amazing. You won't lose much money at all on this purchase, unlike the other bikes posted if you were to buy them new.