In an old Digital Photography book I have, they recommended a program called Genuine Fractals to do what you want. As stated, one cannot simply zoom in and create more pixels. But there are likely heuristics that can be used to estimate the pixels, and render a better image than a simple enlargement.
A Google reveals :
http://www.ononesoftware.com/detail.php?prodLine_id=2
There are some nifty examples there. The software is pricey ($160), but there is a demo version. Not sure what restrictions are on the demo.