Radi0n said:
As far as footage goes, we've only seen 3PO mention Leia, but not actually encounter her. The very moment he walked in on her and R2, she jumped away behind a colon. I'm pretty sure someone as 'unimportant' as 3PO, at least on that particular consular ship, never got to see its consul. Therefore, he probably knew of the Princess, and her history, since it was common talk amongst the crew members, but never got to see her in person, which could perfectly explain why he did not know who the lady in the hologram was. Not to mention he got his memory wiped in ROTS.
I've long thought Threepio is "playing dumb" as he doesn't know where the political sympathies of his new owners lies just yet. He doesn't want to get blasted or melted down. He only loosens up a bit when Luke expresses an interest in the Rebellion and the battles witnessed. And we did see a droid being taken prisoner aboard the Tantive...
Why Leia hides from him is a bit of a mystery though. Maybe she just didn't want an easily to interrogate droid to know where the plans were hidden?