Carrier flashing is not illegal. What kcls means is altering your MEID (also called an ESN), which is at the least technically illegal (note: the purpose of the FCC regulation is to prevent cloning and fraud, not switching carriers) and is not something which you can discuss here.
Virgin Mobile requires to use a phone MEID which is already registered in their database. You can call Virgin and ask them let you register your MEID and the will almost certainly say no. Rumor has it that any virgin MEID will work with Android phones. Don't ask how to change it here It's illegal and against the rules.
Your best bet is to spend the $65 and buy an Optimus V from Best Buy.