If you use Mac OS X, you're comfortable mucking around with really dirty scripts, and you want to change your smtp server for Mail.app every time you change location, try autosmtp what I just wrote in a fit of utter geekness. Filthy, dirty script.
(Can I just point out that I would never be able to do anything like this on Windows or Mac OS Classic. But because Mac OS X and the unix underpinnings are so tightly bolted together, I can use Perl (which is easy, and I know from making websites) to bolt together all the bits I only slightly know, and still make my day-to-day gui existence easier. This is what an OS should be like.)