Later, during the switch from single data rate to DDR, there were some motherboards that took both—you could either use one or the other (but not both).
Now, the switch from 32-bit CPUs to 64-bit CPUs is underway...and someone's making motherboards that take both, so that you can either use one or the other (but not both, I think).
The manufacturer is ASRock, and while I haven't found out too much about them, the concept is very attractive, especially since I read that one of the choices is the 939-pin flavor of Athlon 64, which looks like is the way that AMD is going for into the foreseeable future. I'm therefore seriously tempted to eventually get one, so that I can start with a reasonable Socket A CPU and then move up when the 64s become sufficiently attractive. (The problem is that I can't really justify it after getting myself set up with a decent Socket A mobo last December. I don't really need to be on the bleeding edge.)