This was my international calling set-up about a year ago (note you may struggle to get the AT&T unlimited international data plan these days):

iPhone 4 on AT&T with unlimited international data plan ($65/month on FAN accounts before FAN discount)
AT&T 450 minute voice plan: $30/month before FAN discount)
Skype with $2.95/month unlimited U.S. calling
Skype-In with U.S. number (~$30 for six months, IIRC)

Incoming calls: My AT&T number was forwarded to my Skype-In number. This just uses up your minutes for incoming calls but otherwise incurs no costs from AT&T. Answered calls using Skype application on the iPhone with Push notifications over 3G (or wifi if I happened to be in a wifi hotspot) - note you could also set Skype to forward calls to a local SIM in a different phone
Outgoing calls: Skype for outgoing unlimited calls. Changed Skype settings to show AT&T number as the Caller ID. I was outside of the U.S. continuously from June 2009 until the end of November 2009 and didn't receive a notice that I might be violating AT&T's terms of service until the end of October 2009. AT&T didn't do anything beyond sending the single SMS about it and I continued using the service as I had been (with about 2gb of international roaming data per month) without any repercussions.

If you don't need an AT&T number, you could just give out the Skype number and not even have a voice plan on the iPhone. I think you can also SMS from Skype, but you're charged for it.

I've now slightly modified this in an effort to be even cheaper. I give out my Google Voice number and have Google Voice forward calls to a Gizmo5 (now Google-owned) SIP account. I use the Gizmo5 application on my iPhone for incoming calls internationally (over 3G). I continue making outgoing calls using Skype's $2.95 unlimited calling but it might cost less (or the same) to just use Gizmo5 for outgoing calling depending on how many outgoing minutes you'd use. (I now have Skype set to display my Google Voice number as the number calling on Caller ID.) I use the Google Voice iPhone application for SMS'ing, and it's great.

In the end, I have unlimited incoming calls and unlimited outgoing calls to U.S. numbers from almost anywhere in the world for $68/month plus tax.