A screenshot published by 9to5Mac indicates that Genius Bar staff are being warned about a lack of iPad 2 64GB Wi-Fi devices for replacements. The upside is that Geniuses are reportedly being told to replace faulty units with 3G-versions.
9to5Mac points out that it's probably not a good idea to try and scam yourself a 3G upgrade, though:
Genius Bars are super strict about replacing constrained items.
It's still worth keeping in mind though if yours needs looking at. [9to5Mac]