I guess the first official 'date' would be the interview that Lois scored in the pilot. As we all know, Lex made it dinner and although Lois skipped dessert, Lex still kissed her goodnight.
By IGaCoY, Lois already considered Lex a 'friend' of hers, so I suspect there was at least one more 'date' before then.
B
ETA: Of course, in the episode just before IGaCoY, Lex shoots Max Menken to "save" Lois, so it's possible that she would consider him a friend after that, even if they hadn't seen each other since the pilot.
I agree with Nancy, though. Although Fly Hard is the first time we see Lois and Lex on a date, no one (including Clark) acts like this is a first date. I think you could make a justification for off-screen dates for just about anytime between the pilot and Fly Hard.
