I seem to remember hearing somewhere that the total
memory was something like 128 Mb -- I wouldn't put too much weight on that, I don't know how reliable it is.
Whatever the 'total' memory is, they clearly put the OS/App set into it too, and we get whatever's left over. As noted, right now it's around 90Mb. I think the "64 Mb of User Accessible Memory" thing is more in the area of a statement that they don't plan to fill so much of the internal memory that they leave us less than that 64 Mb.
I guess I said all that to say this: whatever the current usable memory is, I think it's likely to move around some with firmware updates.
That's a lot of speculation in there, so salt to taste.