Hey there Uppercut... Great questions.
I dropped about 10 pounds during phase 1 of P90X. During phase 2 I lost about 5 pounds and then I hit a plateau. I think that is because my weight loss slowed while my lean body weight rose because my body fat measurements were still going down. During phase 3 week 3 my weight just started coming off again... another 10 pounds.
I did not follow the P90X nutrition guide. I had lost about 50 pounds about a year and a half ago so I used my same strategy.
Here is the strategy I used...
1. Burn more calories than I consume.
2. I don't subscribe to low carb diets because I need good quality carbs to fuel extreme workouts. Eat good quality carbs!!!
3. Consume protein to build lean body weight.
4. For most of P90X I used this caloric ratio... 40% Protein/40% carbs/20% fat.
5. Finally, the single most important tool... keep a food log. Enter everything you consume no matter how small. Even those who are good at estimating are normally 20% off.
I did doubles throughout the program.
My calorie intake during P90X...
Phase 1 calories 1200-1400
Phase 2 calories 1500-1700
Phase 3 calories 1800-2000
With this program I lost 25 pounds and went from 22% BF to 12%.
As for how I figured my daily caloric requirements? Check my post about this here...
http://www.projectexercise.com/Blog/post/2009/03/17/Tip-How-to-figure-out-your-BMR-and-Calorie-Requirements.aspx. To lose weight eat about 500 calories or so less than your requirements.
I hope this helps. If you have any further questions, please feel free to ask.