Just to confirm
Our environment we only have a single mac mini that has leopard server installed. we have the media for snow leopard but i've been reading that in order for mobile access server to work you need two servers; one acting as a proxy and the 2nd being the main server inside the firewall.
SO it is possible to run mobile access server on a single server without any additional hardware while still using this single server for other duties like file server, print, etc.
If you could post a link to your source it would be helpful.
I would imagine, as I've not looked in to this for SL Server, that the proxy would, and excuse this, proxy the request/connection.
In essence, it could inspect the traffic to ensure it's legitmate and then either foward the traffic on the internal server. The traffic could appear to come from the server or, more likely, the device. Although this is best practise it does allow for a more secure environment, as normally any sort of proxy would go in a DMZ, with only the required ports being opened on both firewalls.
I would suggest you don't worry about it and just go single server, unless your data is secure, in which case, I would suggest both servers, with a DMZ and proper server architecture.