Only if they make a change to the architecture. When MicroSDs only went to 2GB, they came out with the MicroSDHC standard. While physically they were the same, the internal architecture was slightly different. They could fit virtually every phone that had a MicroSD slot, but most phones hade difficulty recognizing them or reporting the correct size. A driver update fixed that problem.
So unless they change the internal architecture, a 64GB MicroSD card should work fine. They said that the G1 could only hold up to 2-4GB I believe. I have seen them use 16GB cards. Same for the Droid 1 supporting up to 16GB, and I have seen (only 1 so far) a 32GB card work flawlessly with it.
The Droid 1 has always been stated to support 32GB. Not sure why you say 16.