Update: Apple iOS has fully integrated MMS support in the Messages app. The latest firmware also includes support for Apple's iMessage, which can be set up to send messages between iOS devices without using a text plan.
Contrary to popular belief, the answer is most likely YES!... sort of. Web applications are going to have a huge impact on the iPhone. As users will not be able to install their own software, they will resort to web applications. Instant messaging can be achieved through sites like XS4All's web chat site.
Officially, the iPhone will not provide native instant messaging functionality.
Each iPhone owner can receive a free push IMAP email account from Yahoo!. Push e-mail systems provide an "always-on" capability, in which new e-mail is instantly and transferred to the email client from the server. This will be useful as there is no instant messaging software for the iPhone, such as AIM, ICQ, or IRC.
