That's actually a neat idea; I haven't heard of it before. Cisco publishes API documentation on how to invoke the Extension Mobility service:
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_programming_reference_guide_chapter09186a00801ede59.html
With a little programming skill and elbow grease, you could develop code to put in your Windows login scripts to log you into the phone automatically when you login to the PC. Here's what the program or script might look like:
1. Determine what phone is next to that PC. One way would be to have a manually created database of PC hostnames and matching phone hostnames, and have the script look up its own hostname in that database to learn its adjacent phone. Another way would be to write another small program to run on the PC that listens for CDP packets from the phone.
2. Given the username you used to log into the PC, make some calls into the EMobility API. You wrap the login username and the phone hostname into an API call, and there you go. Depending on your policy and whether or not you allow multiple simultaneous logins, it might be a good idea to make a couple of calls first to list the devices they may already be logged into, and issue logout requests for those.
3. In your logout script, send an EMobility API request to log the user out of the phone.
There's obviously error checking to be done, and policy checks to make on which users get this service and which don't, etc, but this is about what it would look like.
One "hole" in this scheme: a phone continues to be logged in if a PC crashes or is abruptly powered off, preventing execution of the logout script. It's up to you if this is a problem or not. You could partially alleviate the problem by setting the phone to auto-logout after 12 hours or so (don't interrupt during the work day, but make sure the a user has to login the next day). A real solution would require a custom service running somewhere that periodically checks to see that user is still logged in and to logout the phone if it can't see that PC anymore or can't see that user logged into that PC.
