Chicago Tribune review of House episodes 1/2-season 5 

So, if Lucas does not appear in episode 6 (in which House and Cuddy "lock lips" if we are to believe Ausiello), it could be that The Kiss is not triggered by jealousy... What do you guys think?

I agree that it won't be triggered by DIRECT jealousy, as in, it won't be that Lucas did something before and therefore House's reaction is to kiss Cuddy. But I DO think that Lucas BEING there will indirectly affect House's decision and that's why there'll be a kiss (so jealousy will be part of the reason but not entirely). This is after all the first time someone has really presented themselves as a threat in the House/Cuddy relationship. House is probably realizing that if he doesn't do something, he might just lose her.

You're probably right, House first needs to see that he can't take whatever he has with Cuddy for granted to make a move, hence the relation of cause and effect between the PI and the kiss. Maybe he needs to see her in a new light, in the eyes of someone who notices for the first time how attractive, sexy, funny, smart, etc. she is.

It's probably a combination of Lucas's crush on Cuddy and what happened with Amber that will cause House to take the leap.