They don't exist yet, he was referring to the fact that documentation says that this new SDXC technology could theoretically support up to 2TB of data. But we currently aren't anywhere near being able to fit it that small yet, but the format supports it.
Im pretty certain it can, yours is no different then any other 16 gig microsd. The New ones are MicroSDXC, and I believe I read somewhere that the Prime does support those, which currently go up to 64 gigs.
http://www.engadget.com/2011/12/19/asus-transformer-prime-pre-order-shipments-delayed-at-best-buy/
Amazon expects new shipment in first week of January.
Just got off the phone with my local Frys, and they said they plan to have some in stock, in store next week. While I take this with a grain of salt considering the 23rd delay and now the mid-January delay floating around.