From what I've been reading, Socket A is probably going to stay around for awhile. AMD kept the Socket 7 alive for several years with the K6's, and so far, AMD has been one to stick to a particular socket for several years, and even past when it releases it's new line of CPU's. A64 will be different as there are a few different socket types, which are going to establish the consumer Athlon64's from the Opteron's.
The other thing that's going to keep them from the mainstream is the price. I own 3 PC's, but I also don't buy the lastest and greatest the instant it comes out (don't we all wish). For most people, the price of the A64 is too high to justify the cost. The biggest advantage with 64-bit computing is the ability to natively address over 4GB of memory. It is done with some high-end server motherboards, but that's through a combination of hardware and software. 64-bit raises the cealing to new levels...the exact number I'm not sure of, but it is quite high.
Harrack pretty much explained the other things that'll keep it from the mainstream market for awhile. I wouldn't be at all surprised if we don't see A64 in the mainstream until this time next year.