Just a quick reminder. You can instant message between iChat/AIM and the iPhone (or any SMS capable phone) by starting an iChat conversation with any US/Canada cell phone number (+1-xxx-xxx-xxxx). Response time is amazingly fast.
In iChat, click the + at the bottom left of your buddy list and then click the New Person button. Simply add their phone number (with the +1) as their Instant Messaging user name and your all set.
The phone number must appear in your buddy list with the leading "+":