Then the "dynamic user tracking" feature of Campus Manager 5.x may be of interest to you:
http://www.cisco.com/en/US/docs/net_mgmt/ciscoworks_campus_manager/5.0/user/guide/whatsnew.html
It does have several prerequisites such as enabling CISCO-MAC-ADDRESS-NOTIFICATION-MIB traps on your Cisco devices (not necessarily all of which has this support), enabling DHCP snooping, and deploying UTLite on Windows workstations.
If Dynamic UT isn't "real-time" enough for you, you may also want to check out the SwitchPortMapper.exe utility in Solarwinds Engineer's Toolset that does what the name says, as often as you're willing to run the discovery.