GlobalSign's CA is inlucded in iOS and Mac OS X.
Keep in mind that certificate messages during initial authentication to an 802.1X network are not system certificate trust related, they are to prove the server identity to the user connecting. Server certificate validation is a normal component of tunneled EAP methods.
The only ways to avoid that message on devices are:
1) Move to EAP-TLS (ideal)
2) Offer a configuration tool like QuickConnect to users
3) Push down configuration on managed devices (GPO or Profile Manager)
4) Manually configure supplicants.
If you're going to Atmosphere, we'll be discussing this in the Deploying Device and Server Certificates session.