The fact of the matter is, a 32 GB microSD card is the largest size for SDHC (Secure Digital High Capacity) cards. The SD Association has come out with a new format called SDXC (Secure Digital eXtended Capacity) which can hold from 64GB to 2TB. Whenever microSDXC cards become affordable enough, they won't work with most smartphones today, because the ports aren't designed to handle XC cards.
If you remember, there were some feature phones that could only use microSD cards with a 4 or 8 GB storage capacity. I really doubt most cell phone users today, need more than 8GB (or 32GB for smartphone users) of external storage. Perhaps when we're using our smartphones as our main computer then I could happen.... And I'd be the first person in line waiting for that kind of phone.
EDIT: and when I mean new, I mean they revealed it CES 2009... so yeah I think we're still a bit aways from a 64GB microSDXC card.