I haven't heard of this being done from sprint to virgin yet but I think sprint recently bought virgin so that may improve your chances.
It's almost as if virgin's become the pay-as-go arm of the sprint giant.
I have been following reports of people getting their sprint evo working on boost mobile...
HTC EVO 4G Sprint to Boost
I'd be interested in setting up my no-contract evo with the option for pay-as-you-go data, but so far it sounds either too complicated or shady.
Please keep us up to date with with your findings. I'm pretty much right behind you.