There's been optimistic predictions that Skype will be allowed over 3G like iCall for a while now. It's been too long. I want Skype over 3G!
I too use Skype when I'm overseas and calls to US numbers work great. The great thing about VOIP is that you don't have to edit the outgoing phone numbers as long as you have them stored with +1(234)567-8900, +56 99 555-1212, etc.
btw, if you use Skype frequently, then you might want to change your QoS setting to "high priority", so it gives Skype calls preference over other processes over the same router. Otherwise, you can experience poor call quality.