One of the dozens of threads covering this should be stickied.
You cannot activate a non Virgin Mobile phone with Virgin Mobile because they will refuse to activate any phone that lacks a Virgin Mobile ESN. No matter what you do to your phone (flash roms, PRL's, place a voodoo curse on it), there is no easy way to the change the ESN. There are also legal issues due to FCC regulations (some may disagree). Bottom line, you need a Virgin Mobile phone to use Virgin Mobile.
You have two options:
1. Buy an LG Optimus V and use Virgin Mobile.
2. Buy an LG Optimus S and use Sprint or Alltel.