Reporters are conflating text messaging, instant messaging: I keep seeing articles like this one from the Washington Post, in which it states that Aircell's in-flight Internet service won't allow voice calls but will allow a variety of Internet services, like instant messaging (IM), as well as text messaging. That's not quite correct.
SMS messages require a cell network to back them when sent and received using a carrier's system. There are SMS gateways, including a free one built into AOL Instant Messenger, that allows a gateway user to avoid fees for sending or receiving IMs, or to use a fixed-rate plan (depending on the service) via an Internet connection. That includes services like Truphone which allow SMS and VoIP over WiFi, although VoIP will be blocked.
Aircell has no current plans to put picocells on board, that would act as mini cell towers, and that would be required for native SMS. The fact that people can use gateways and workarounds still doesn't allow them to continue to use an understood mechanism while in flight. People will need to change behavior.