Well I've setup Snow Leopard Server on my Mac Mini, I got RADIUS working with my Airport and users were able to connect however the internet side will no longer flow through. Any ideas? Could it be because the server controls the Airport it controls the DNS or DHCP functions?

How is DNS & DHCP controlled normally on your network?
What are you using as the RADIUS client?

I'm using snow leopard server on the mac mini to run radius which seemed a simple setup, os x server take control of the airport extreme and configures it for radius. Because os x server takes control of the airport I was thinking that maybe the server wants to control dhcp or dns.

Currently it's a simple setup the airport controls dns and dhcp automatically.

I've not setup static IP addresses for anything just FYI.

Server's should be on a static address anyway.
Are you using RADIUS to authenticate users requesting network access or for VPN's?

Initially radius for WPA enterprise authentication but VPN at a later date.

I tried setting my MacBook pro with a static IP but the airport didn't like it. And I didn't look further into to sorting that. But it's now on my list.

I'd put the server on a static but you can leave the rest on DHCP.

When the devices connect to wi-fi & authenticate on the network so they get an IP address through DHCP or do they go to apipa mode?

Is there a list of users (with matching passwords) on the server?

I've setup temp users with accounts which authenticate ok. I think you may have hit the nail on the head I'm not sure it picked up an IP which means it's not connecting to a DHCP, right? APIPA mode?

I think when trying to setup Elektron radius I had the same issues and it couldn't connect to the net, when looking in network preferences it couldn't get an IP address so it generated it's own, as far as I can remember.

could it have anything to do with the Airport being set to Bridge mode?

That's APIPA (169.x.x.x address)

Is the airport used just as a access point or as a gateway/router & AP?

Unsure. I have a router from the ISP with the wireless turned off and connects to the airport base station via Ethernet, and have the router in bridge mode.

What's the wan & lan IP's on the airport?

192.168.1.254 is the ISP router the Airport base station is 192.168.1.72

Things I changed, disabled back to mac and setup a static IP on the server with the correct DNS settings.

Goes to show check check and check the all IP addresses are correct.

Thank you Saspro for all your help.