Three quick feature suggestions:
1. If I enter a 7-digit number instead of 10, have the app automatically fill in a default area code.
2. This is probably not possible with a web-app, but if you could make the touch-tones sound as you dial, that would be cool.
3. The ability to paste a phone number from the clipboard to be dialed is very handy when you are grabbing phone numbers off of web-searches. Just a "Paste" button on the dialer screen would work (if that's not possible with a web-app, having a text-box somewhere that you can paste into would do the trick even if on a different page as the dialer).