I have a coworker who will likely be leaving the company soon. An msdn subscription (Premium I believe) was purchased for him - but I believe it was more than 1 year ago. Is this license transferrable to me when he leaves the company?
It should be transferable to another person:
If you didn't use volume licensing and bought him an individual subscription, it can likely still be transferred to a new person, although depending on how it was obtained there may be exceptions.
Typically, software accessed through an MSDN subscription comes with perpetual use rights, which means that you're allowed to use it after the subscription expires although you will no longer have access to the keys and downloads after the expiration date. These rights are transferred along to the new person who will be using the subscription:
There are some exceptions to the above, such as with software obtained through the Microsoft Partner Network and these terms may change over time. I also may not have covered your specific situation since you didn't mention what kind of subscription was purchased or how it was done, so be sure to read up on the current licensing terms over at MSDN Licensing. The quotes above are from the Visual Studio 2010 and MSDN Licensing White Paper.
For instructions on how to do the license transfer and to get a definitive answer on any licensing questions, contact Microsoft's MSDN support.