My theory about Regina's potion from Save Henry was that it erased all of her fairy tale memories, and while she was raising Henry, she was "cursed". However, in the end of Pilot, when she was holding the book and looking worried, she got them back. Thoughts?
She drank the potion in her secret underground magic bunker. She only forgot about finding out that Snow White's daughter was Henry's mother, not that she's the Evil Queen. Otherwise she would've wondered where she was and why there was a secret entrance to the magic bunker under her father's casket.
I believe the potion made her actually forget finding out the savior is Henry's mother and wiping that information clean from her mind. It makes sense since the scene prior with Archie was her expressing fears the birth mother would inevitably, by fate, come back for Henry. She already has Rumplestiltskin's prophecy hanging over her head about the savior who will break the curse in twenty-eight years (well, actually 10 more years since the information on the faxed paper said Emma is already 18 yrs old). I belive it is stressing Regina out even more to know that person is her child's birth mother and that when the time comes, she'll not only lose her status and power, but also the one person she loves (Henry).