I just got off of the phone with a Boost Customer Rep. and they told me that if you were to purchase a Sprint branded LG Marquee that it would activate on the Boost Mobile network. He said that the only thing that would not work would be multi-media messages. You would still be able to place/receive calls, regular text messages would still work, along with the Android Market and 3G and such. Your phone would receive MMS but it would not be able to open the message. You would have to forward the message to an E-mail account in order to actually retrieve the full message.
They also said that if you are upgrading your Andriod device from one that was grandfathered into the $50 unlimited that your monthly payment would go up the $5 due to the upgrade.
Hope this helps you out!