Keep in mind that the Droid uses (AFAIK) a pretty ordinary SD card. So yes, you're constrained by the expansion of SSD storage, but able to move as quickly as it does, and ought to be able to upgrade when 64GB comes out.

(Admittedly, I haven't looked into this closely -- it's going to be some time before it can replace my 120GB iPod, and the 16GB it comes with is plenty for my other purposes...)