Since we put our Apple Caching Server in to place we have had scattered reports of Software Updates not working on some wireless clients. After a bit of investigation we found that AppStore and Software Update will time-out when it finds an app or update that is already stored on our local caching server only if the client is on CAGuest.
CAGuest is a restricted wireless network that only allows traffic on ports 80 and 443. That basically means that usage is restricted to web browsers (port 80 is standard WWW traffic and port 443 is encrypted traffic). Software Update and the App Store use these ports along with a third port, 8088, to communicate with local Caching Servers.
Essentially Apple's servers will tell the device (MacBook Pro, iPhone, iPad, etc) to ask the Caching Server when it is requesting an update that has already been downloaded. The device will be able to see the Caching Server but not be able to initiate the connection to download the update. Since there is some communication the update will eventually timeout instead of giving an immediate error.
For campus visitors and guests we really don't have a solution to this issue. For Students, Faculty, and Staff, however, CASecure should be used (with your MyCalArts credentials) as it doesn't have any of the restrictions CAGuest is saddled with.