Quick and interesting little gotcha for those running or upgrading any systems running trail, evaluation or time-limited licenses out there.
If you try and add a host to vCenter and the agent replies with an error reporting that the evaluation license has expired, try checking the BIOS time and date settings for your ESXi host. If they are anything other than close to the vCenter system time, then this will affect your evaluation period and may result in the timeout error.
Solution: Reboot your ESXi host and check / update the BIOS date and time. Problem Fixed!
This solution is also detailed in VMware KB 2011655 here: