Just to clear this up for people you don't need to purchase 3G Unrestrictor when it comes out in order to get this to work. If you check out crash-x's page @ VoIPover3G and view the change log you'll see he's already written version 1.1 to be released that will support 3GS, the advantage to 3G Unrestrictor is that it make the configuration easy and I believe the list price was to be $2 so you're not going to break the bank.
p.s. I believe from his tweets we should be expecting both within the next day or so.
VOIPOver3G v1.1 released Yesterday. I have tried today and it works now. With Skype and other applications, you can now going over 3G instead of using wifi.