Will messages sent via iMessage use up my text messages?
A lot of people have been asking this question. The simple answer is no.
iMessage messages are not sent via your wireless provider, whether it be Verizon, AT&T, Orange or any other provider. These messages are managed and delivered by Apple, and as such have no impact on your text (SMS) or multimedia (MMS) messaging plans through your provider.
Please note, however, that messages sent via the Messages app on the iPhone to users that are not iOS 5 device owners will count against your monthly SMS / MMS message limits.