Hubble is an awesome 'scope, but its life is limited. Heavy and huge, there's no way to bring it back, and with the Shuttle retiring there's no easy way to get to it. Eventually its gyros will fail, it won't be able to be pointed, and then that's that.
For the past few years, NASA has been working on the James Webb Space Telescope, what some people call Hubble's "replacement", which is a misnomer: it's actually Hubble's successor. It will do amazing astronomy too, but it has different capabilities than its predecessor.
How different, you ask? I'm glad you did, and so is NASA: they've put together a side-by-side comparison of the two observatories (warning huge Flash animation stuff).
Just how different are they? Check out the comparison of their mirrors:
Hmmm, the woman in the diagram is pretty tall, 1.8 meters -- 6 feet! Of course, she's in heels. But should she really have her hand on that priceless (if incorrectly ground) mirror?
Anyway, check out the comparison. I'll miss Hubble when it goes, but I'm very excited about what JWST will do for astronomy, for science, and for humanity's search for understanding. It will be a powerful, powerful tool.