Are you saying Leia and R2 had some "secret" relationship?With your idea in mind, perhaps 3PO and R2 didn't really work together a whole lot between ROTS and ANH - they could easily have been assigned to the same task/mission only shortly before we see them at the beginning of the movie.
It seems to me that perhaps R2 has more to do with Leia - entrusting secret Rebel mission data and Death Star plans to a generic/random droid doesn't seem like a mistake that Leia would make. I always figured R2 was attached to her before the movie starts.
On a side note, she did know there was a tracking device onboard the Falcon and headed for Yavin anyway, leading The Empire directly to them. Perhaps she figured the plans were more important to deliver than actually jumping around from system to system whilst they deactivated the tracking equipment etc.
Interesting. I never really considered that R2 is basically a giant mechanized phallus.