Client Access Licenses (CALs) are usually purchased as User CALs or Device CALs. Up until now, the pricing for these have been the same. But on December 1, 2012 Microsoft will be increasing the price of User CALs by 15%. And it makes sense, I suppose.
Device CALs are primarily useful, in my view, for shift work scenarios, where multiple people will use the same computer. E.g. John uses it from 6a-3pm, Sally uses it from 3pm-11pm, and Dave uses it from 11pm-6am. In these environment, one device CAL is the better (price) choice.
User CALs allow the same user to connect to a server from multiple workstations/devices. The price increase is in response to the “increasing value of user based licensing over the single device licensing model”.
For more information, check out:
http://www.softcat.com/news/industry-news/microsoft-client-access-licensing-rules-to-change