When you send an IM, isn't that considering sending a text to VZW? When I used an IM program on my devour, it always warned me that significant charges could occur without a texting plan... so I assumed that it was like sending a text, only using the IM program... I could be very wrong though, and it didnt matter to me anyway as I had the unlimited texting plan already. Would be interesting to know though.
A text message use the SMS protocol and is a different animal.
An IM on a smartphone is using nothing but data. Just like an email or webmail or whatever.
Perhaps you are thinking of an SMS app, like Handcent or ChompSMS? SMS is texting, so those are texting apps and standard texting rates apply.