I've had an Altec Lansing inMotion iM600 speaker system for almost half a year now, and I'm very pleased with it. It supports older iPods, like the iPod Nano I used to have; the sound quality is very good; and I love that it can run on either AC power (i.e., plugged into an electrical wall socket) or you can disconnect it and run it off the battery for a very long time -- it hasn't died on me yet during normal use.
My Altec Lansing inMotion iM600 product review has been moved here.
Whenever I plug my iPhone 3G into accessories that I used previously with my old iPod -- including an Altec Lansing iM600 external speaker system and a JVC car stereo -- I get the scary message "This accessory is not made to work with iPhone" (followed by a message about switching to airplane mode). That message is almost mind-blowing scary like "Your iPhone is going to be fried if you stay plugged in to this device". Is it really that bad?