I just finished setting up the three little pigs siri proxy on my ubuntu server 10.10. Its a public server, but I might have an idea to make it private. I want to be able to use the server on wifi and 3g so I know I can't only accept certain ip address on my servers. Correct me if I am wrong but mac addresses don't change right? So my question is how could I block all mac addresses except the ones of the devices that are allowed to use my server? Is something like that possible. Could someone recommend a better solution? I am new to linux.

MAC addresses are used on layer 2 and mean nothing outside a LAN. You can't use them for internet traffic.

how could I do it then?

I don't know what a siri proxy server is. You will probably need to dig in its documentation to see what kind of authentication it uses. Maybe it is key or certificate based and you need to create and copy keys. Another possibility is a vpn tunnel, which would also involve certificates.

If by wifi, you are referring to your own wlan, then you can use fixed IP addresses or MAC addresses to filter access. If you meant the client may be using public wifi to access the network, then you can't. The wireless router will probably be nat'ed
so there will be several devices with the same IP source address.

You will either want the server located on the edge or in the DMZ to prevent unauthorized traffic from existing inside your LAN.