Thanks, should have mentioned that my interest is mostly astronomy related.
In that case, forget entirely about SLR lenses. Instead, get yourself a decent (real!) astronomical telescope and whatever adapter you need to mount your camera to it.
Everything involves compromise. There are lots of compromises necessary for making photographic lenses that come at the expense of astronomical suitability, and vice-versa. An upside-down image is a non-starter for a photographic lens, but hardly even worth mentioning for a telescopic lens, for example. Pleasing (or, at least, not distracting) out-of-focus blur is necessary for a photographic lens, but not even remotely on the radar for astronomers.
You will get <i>far</i> more bang for your buck with an astronomical telescope than with a telephoto lens.
There are exceptions, of course...if you're looking for wide-angle shots, such as of the entire Milky Way, then you'll need a typical wide-angle photographic lens. And a Great White is going to do a pretty decent job at wide-field astrophotography...but a $12,000 telescope is going to do a much better job than a $12,000 400 f/2.8 II. Still, if you're wanting to do an automated search for extra-solar planets, you're probably going to go with an array of several 200 f/1.8s.http://en.wikipedia.org/wiki/SuperWASP