According to a debug screenshot BGR acquired last month, the 1GB spec was just hiding in the numbers:
It's been in plain sight since the beginning of February when BGR posted their exclusive photos of debug output from an iPad 3 development unit. The (full) last line says:
vm_page_bootstrap: 244276 free pages and 8396 wired pages
So, just do some simple math:
244,276 (pages) x 4,000 bytes (size of a page) = 977,104,000 bytes = 977MB
Gibberish to most, but if you take his word for it, that last figure is the amount of memory the iPad 3 will pack, so round on up to 1GB.