HST lifespan
There are some discussions about returning the HST in 2010 (or at least, when the NGST will replace it).
But why not, during its last servicing mission, add a booster to it and send it slowly to a much higher orbit (like 1,000 km or just below the Van Allen belt), and leave it operating normally until it crashes in the atmosphere?
Operating the HST surely costs much less than developing, building and sending new instruments. Even returning it will cost much more than letting it go on providing us with beautiful pictures until the very end.
Raoul