if both data and wi-fi are enabled, will iPhone use both?
If you just saw that you exceeded your data, it could've been many hours ago while you were not at home. AT&amp;T says it could be delayed by 24 to 48 hours.

In my experience, the iPhone will only use data from one connection at a time, not both; Its choice is indicated by which adapter symbol is showing in the menu bar. When you see the Wi-Fi symbol, your iPhone is only using Wi-Fi for the internet.

Example: When I'm connected to a really bad Wi-Fi network (if I am almost out of range or it lost its internet connection), and trying to load a page in Safari, my iPhone will struggle with loading the page for about 10 to 20 seconds. Then, the Wi-Fi symbol switches to the 3G symbol and the page loads using cellular data.

The only exceptions I know of are multimedia messages (MMS) that aren't iMessages and Visual Voicemail – both of these things will not come over Wi-Fi and can't be received with cellular data off. They are not using both, but they could use cellular while you're on Wi-Fi.

Last bit of evidence and future-proofing: In iOS 6 Beta 4, Apple added an option for "Wi-Fi Plus Cellular". (MacRumors: iOS 6 Beta Adds 'Wi-Fi Plus Cellular' Option...)

It will help people who may be on a Wi-Fi network with little or no internet connection be able to get cellular data through to some apps. (Maybe I'm just on the Wi-Fi network for Airplay for example.) It allows you to choose (from their pre-set list) which apps can access cellular data while you're on Wi-Fi. Before iOS 6, this option was unavailable.