I've been trying to figure this out too... I think the VPN is the only way to get it working over 3g, as there's no way to override the DNS on the cell connection. Thanks for that idea, VPN hadn't occurred to me! I'm going to give it a shot; I'll report back if successful.

Edit: Success! I'm not running it on EC2 yet, just my home network. Basically I've got dnsmasq on my router redirecting guzzoni.apple.com to my home fileserver with SiriProxy running. The router is also running a PPTP VPN server. I can use the VPN over 3g to make Siri requests and see the output on my local machine. Thanks again for the VPN idea; that's gold. Next step - to the cloud!