If you are watching Lost on Hulu.com or thru Hulu desktop.. read this from this page :

We are able to post all episodes of the current season until the finale on May 23. At that time, we will be able to post the last 5 episodes of the current season.
Seasons 1-5 will be available for streaming through December 31, 2010.

Thank goodness I got caught up a few weeks ago!