I have always wanted card entry systems to know what kind of card it is - given that it's trivial to tell from the first few digits I never understood why not.

This is a great step up from that. The one thing I felt was not perfect was the multiple entry boxes for the different card types. Erasing/backspacing is not so easy. I would suggest a single entry field, and do something about adding padding spaces automagically. This also plays in to the auto-complete/form fillers these days. A credit card number is a single number, not 4 groups of numbers you copy and paste independently.