HTML 5 is coming whether I like it or not, and in order to familiarize myself with some of its new functions, as well as to try out a couple different programming techniques, I built a simple web application to play blackjack.
There's no splitting, no insurance, and you can see the dealer's hand from the beginning, but the whole thing comes in at under 50 KB, stores player data with a client-side database, and can be played on iPhone as an offline app.
It works on the latest versions of Safari, Chrome and Firefox, though Firefox will display a mildly annoying warning about storing client-side data. Internet Explorer is right out. Maybe by IE 9 they'll follow the same standards as everyone else. I haven't had a chance to test it in Opera yet.
To play offline on your iPhone, simply hit the + button and choose Add to Home Screen.