Are you sure you're not thinking "copy on write" rather than "zero copy"? The latter implies you can predict in advance which pages will be the same forever...
>>andrew+(OP)
The pages would be copy-on-write, but since this would mostly be for code pages, they would never be written, and therefore never copied.
By 'zero copy', I mean that when a guest tries to read a page, if another guest has that page in RAM, then no copy operation is done to get it into the memory space of the 2nd guest.