All that has happened is NTL:Telewest have taken over Virgin Mobile and decided to rebrand the entire company Virgin Media. As part of the deal Richard Branson owns 10.7% of the company, hence why everything your getting through the post is from him!
Nothing is going to change apart from the company name on the top of your bill. You will continue to use the same modem (though new Virgin branded ones will come out, but they are basically the same as existing ones only black and have no USB port) and I wouldn't expect the service to change much either, at least not right away, though it should improve over time.