At the end of last nights episode of Lost (episode 19 of season 3), the promo guy said something about how "an end date had been set". Thinking this was just channel 7 garbage I dismissed it. But then got to thinking, so I decided to check the fountain of wisdom, Wikipedia. This is what I found:

In May 2007 it was announced that Lost will continue for a fourth, fifth, and sixth year, concluding with the 117th produced episode in May 2010. These three final seasons will consist of 16 episodes each, running weekly in the spring uninterrupted by repeats.

So, yes, an end date has been set, but it is quite a way off.