Yep, it is not a trivial exercise to get the information - basically you will need to write a network device driver hook (and all the management that entails) to sniff out the CDP/LLDP packets and then parse the contents. I did a bit of research awhile back that indicated winpcap (a generic library for sniffing network packets, with all the security considerations that entails) might be a good place to start.
Yea, this doesn't exactly seem trivial/straight forward... I was looking into doing this myself, but the CDP/LLDP packets required only pop up every 60 seconds.
I found this tool
http://www.hanewin.de/lldp-d.htm - 'haneWIN LLDP Agent for Windows' - which seems to be exactly what I want. I pop open the software, and it shows me the info I need to know - what I want to know now is, how is this software polling for this? I was looking at WireShark while it was running, and I clearly see a 'CDP' packet which has all the juicy details.
In conjunction with some AHK and this - http://www.gossamer-threads.com/lists/cisco/voip/145319#145319 - I'll be able to build my own extension mobility login app.
All we really want is an easy way for users to be able to use the PC to log themselves in and out of Extension Mobility - especially if we're planning on going to AD, users are not going to be happy about putting their entire AD username using the TUI. Thanks!