Developers often do more than simply compile code. For example if I could cut the start up time of the app I'm developing in half that would be really quite nice indeed, even if compile times stay the same.
I don't know about that, it depends on the build tree I guess. Our IDE build tree (which includes compiler, RTL and lots of other stuff) writes about 600M of intermediate files, 1G of executables / debugger symbol files / map files etc., and reads probably several G worth of source. A rebuild (clean and build) currently runs in about 13 minutes on my 10K drive, but I'm fairly sure it could be cut down with faster I/O.
Actually Spolsky had already found that to be true for their compile times back in March. But decided the benefit was still worth it for his developers.
See http://www.joelonsoftware.com/items/2009/03/27.html
SSD for developers is usually waste of money from technical POV, as the working set usually fits in the fs page cache.
30" IPS panels, OTOH, would be money well spent.