I'm going to bandwagon on tc1675 and chub's comments a bit. I'm also doing Phase I (Fat Burner) and am a Level 2. I don't have a lot of actual weight I want to lose as far as the scale goes, but do need to drop some padding and replace it with more lean muscle mass (i.e. gain weight). I'm just starting week 2 at the moment.
Regarding the carbs, with only one serving per day, I like to break that up into half portions or even quarter portions, to spread them out over the day. It gives a little extra variety to each of your meals, and helps to keep your energy levels more steady. Just be sure you don't go over the portion sizes, and things so work out well.
Regarding the soda, I haven't read much in the way of studies on diet soda in particular, but that's the sort of thing you can subject to a pretty simple analysis tool--so long as you're willing to be completely honest with yourself. Simply ask yourself the following questions:
"Would I be healthier if I continued to drink lots of diet soda every day? Or not?"
P90X is all about change--and they're all good changes!