The unlockable developer version of the Verizon Samsung Galaxy S3, which just went on sale yesterday, already is on backorder. That means it either was popular as hell, or there weren't all that many available in the first place.
Both the 16- and 32-gigabyte versions are no longer available. There's no telling for how long, but the good news is if you just can't wait, the retail version can be hacked open pretty easily.
More: Verizon Galaxy S3 forums