Has iMessage changed anything about how texts are forwarded from iChat to offline users?

For some time it has been possible to configure iChat accounts so that iChat messages sent when one is offline are forwarded to a mobile number. This conveniently allows iChat-to-moble conversations.

Has the deployment if iMessage changed any of this functionality?

No, iMessages has not changed any of these features. Forwarding to a cell phone is a feature that is handled via AIM rather than iChat.

Furthermore, the new Messages application has not changed any of this functionality either. It works exactly as it did before. It shows a little "wi-fi" symbol next to buddies that are offline, but have AIM Forwarding enabled. However, this symbol is only visible in the Buddies window, and not in the Messages window (the window where you chat). They appear like any other offline user in the Messages window. Therefore, it might help if you remember who does and who doesn't have this feature enabled.

When you send them a message, it goes to their cell phone, and when they reply, you see it in Messages. Note that if they are on invisible mode on AIM, it goes to their AIM rather than their cell phone. This provides further evidence that AIM is in charge of forwarding to the phone rather than iChat/Messages.