You might remember in the early Athlon and Pentium III days, Slot and Socket processor lines were released in parallel. Back then, it made sense to keep compatibility with an older platform that was to be phased out and a new platform that could gain popularity and become the standard eventually.
We are not talking of the same case here, as Athlon 64s will launch with a new platform for itself, and Opteron boards do not match the same core audience... for me, this translates into troublesome upgrades in the long-run, whichever is the socket that stays.
Update: TS reader, Scott Reams, sent an email shortly after I posted this story, mixing some of his views with the rumours regarding an Athlon 64-based 'Duron'.
According to those rumours, the lower pin-count A64 would constitute a lower-end, budget single-channel model, while 940 pin Athlon64s will be dual-channel which give us a good hint on how close and related A64s are to Opterons.