Apparently iMessages prefers cellular data over WiFi. It's been this way since its inception: it is more secure over "3G" than WiFi. It will fall back to on WiFi if cellular is inadequate.
"First off, what’s surprising in the case of the iPhone is that iMessage appears to prioritize cellular data for strictly text delivery"
Source:
http://www.anandtech.com/show/4956/apple-ios-5-review/3
Just to add: I've easily sent and receive over 2,000+ iMessages this past month and have not exceeded 19MB of cellular data. And note that 19MB of data includes other stuff like web browsing and other usage.