Brian,
The HomeLink "Vehicle Interface" is a Homelink X10 transceiver for homes that do not already have a X10 transceiver. If you already have a TM751, RR501, or CM15A Home Link works fine with out the HomeLink Vehicle Interface.
I have HomeLink in one car and contrary to what Dan states, HomeLink is completely compatible with X10. It opens our garage door, and triggers macros via Homeseer. I use the CM15A as my transceiver.
HomeLink is really neat in that it scans and identifies the remotes (X10, garage door, etc) transmit frequency (this is the slow blink training), then analyzes and emulates the modulated code on the carrier (the fast blink training).
Gadgets,
I was successful programming HomeLink when I followed the instructions in the vehicle manual exactly up to the final point when the indicator light for the Homelink button you are training, changes from slow blink to fast blink. My vehicle instructions instructions say the button is now trained, implying you can release the buttons (which is wrong). Don't release either HomeLink or X10 buttons yet. Keep them held until the fast flashing HomeLink indicator quits blinking completely, then it is trained.