Depends on whether you're talking about the flash or the overall effect.
The flash part is easy ... there are a myriad of solutions out there already - all you need to do is properly position it on your page.
The overall effect though is something a little different - and really depends on your budget I suppose.
You just need a good video production company who are experienced with shooting and more importantly lighting a bluescreen shoot - hopefully they have the chroma keying software as well so you don't have to worry about the hassle. Get them to edit and burn it to a DVD for you while you're there, and then all you need to do is convert the video to FLV or something similar to feed your flash app.
You'd get away with it for $5K - $10K depending on the length of your script and the 'princess' factor of your actor / presenter.