I just started p90x and I am wondering if I should do the lean version? I am 24yrs female 5 foor 4.5 and 138 pounds and am hoping to get lean muscle definition while not putting on any more weight. also is there a specific diet plan for the lean program that is different? you advice would be appreciated! thanks =)

If you're not aiming to get bulging muscles then lean is for you. I'm not sure if there's a specific plan for the lean program though.

I totally disagree, you will not get "bulging muscles". I am doing Classic, and am continuing to get smaller. HOWEVER, when you say you don't want to gain weight, do you have any weight to lose? I gained 5lb, but I lost 4% bodyfat and that 5lb was all muscle. I went down 1 size, and am close to being down 2 sizes. I started at 140lb, 5'7", and am now 145lb. I was close to my goal weight-wise, and so the muscle I gained cancelled out the fat that I lost. I had to let go of the number on the scale. I am in Round 2 day 160.

There is no specific diet plan for lean, just follow the regular plan

Agreed. P90X is not exactly synonymous with "bulging muscles". It's more for that ripped look IMO. I don't think you have anything to fear in that regard by going "Classic". Look at the women in the resistance videos. They're all grads and not one is even close to bulky if you ask me.
My wife has this fear of resistance training. She thinks she's going to turn into a Ms. Olympia contestant overnight so she just doesn't do weights. It drives me nuts. It doesn't help that she goes to a women-only gym where many of her peers have this same mis-guided notion. Heck, even the so-called trainer there suggested she only do cardio. Argh!! Sorry for going off on a tangent.

Do the classic. Listen to Tony, he says, for size do more weight, less reps. For lean, do smaller weight, more reps. Why is this so hard for people to understand? It takes muscle to burn fat.

I think it may be confusing to newbies because Tony himself said that many women like the Lean version which basically has a little more cardio, and substitutes Core Synergistics for one of the resistance/ab ripper X days. I started doing Lean for Phase 1, tried a week of Classic for Phase 2, then returned back to Lean for the rest of my first round. I thought I'd prefer the extra resistance over Core Synergistics... but found I kinda missed Core Syn (go figure). Maybe I'll do Classic for Round 2. But really, it all depends on what you want to do.