Really cool! (See my comments on YouTube)
A bug I noticed:
Having the page automatically factor any number in real-time as it's typed is cool in theory, but it makes it a little too easy to crash the page. All you have to do is mash the number keys until you've created an enormous prime number, or at least a number with at least one prime factor greater than what the page can check. (I used 1389537541)
One solution would be to calculate the prime factorization asynchronously, and maybe display the raw number in a different color to indicate that the computer is still working on factoring it in the background. (Actually implementing this in JavaScript may prove tricky.)