I've never used this, but it is an option - point your iPhone here:
http://s4iphone.com/index_iphone.jsp
You have to use SkypeOut credits from what I understand and this web based app costs $25 - there's a monthly option as well.
There are some IM apps as well, but again they are web based . . . for now.
Rumor has it that native apps may be announced around Leopard's launch, which is heavily rumored to be Oct. 26.
I hope you got the unlimited data plan @ $20/month!
Best,
Kurt