Wow, I was just about to start a thread on this subject when I saw your post!
I am having an issue with onboarded MacBooks authenticating with EAP-TLS to ClearPass 6.3. This issue appears to be isolated to MacBooks running 10.8 and 10.9 - other onboarded devices (iPads, iPhones, Android) have not exhibited this issue.
The MacBooks are frequently failing to authenticate with EAP-TLS after being onboarded. ClearPass shows the authentication request as a timeout, giving the Error Code 9002 and the message "Client did not complete EAP transaction".
Packet capture shows that the initial EAP identity request and respone go through, the AP then sends the EAP-TLS/Start message and the MacBook does not respond with the TLS Client-Hello. Shortly after, the MacBook sends a disassociate frame. The frustrating thing is that often the MacBook will then immediately reassociate and perform a successful EAP-TLS authentication!
This is not the result of the client moving out range - the MacBook I was testing with was stationary and in the same room as the AP it was associated to.
This seems like it could be an issue with Apple's supplicant (would not be the first), but is rather inconsistant. Some MacBooks have the issue, others do not.