It's my understanding that Server licences are per user and not per device. One of the core benefits of using server is to be able to access from anywhere.
The current EULA agreeement is here http://mkt.tableau.com/files/tableau_eula.pdf and the section referenceing server is here
Under the Desktop section (2.2) it specifically mentions licence can be used on 2 installs, but for User based Server Licence (2.3.1) there is no specific reference to devices.
Concurrency shouldn't be an issue, as the licence is all about authenticating the access I believe.
Concur with you totally! Donna.
Thanks much for clarifying.
FYI: Tableau does monitor concurrency in their licensing check. So if you're thinking of sharing licenses that's a legal NO NO.