If you haven't seen it already (and I'm sure most of you have) BurnoutAholics' own Xandu has made good use of the extensibility of ILM and created a Race Checkpoints Map - and quite effectively at that. Xandu even has a map demonstrating a custom race.
You too can create your own race and share them with others. All it takes is a little XML. If you find XML intimidating, don't worry. I've been working with Xandu directly and I think we can come up with a way to make it easier for people to make and share their own custom maps without having to write any XML at all. Stay tuned.
As usual, the map has been given its daily regiment of updates. The main Burnout Interactive Map page has a list of all the new map updates. There haven't been many today; I'm spending a bit of time working on a list to throw the individual map sets in and then have another one available for all of the locations within it. At first this will probably be thrown in the Locations dialog UI until I can figure out a better way to present it.
[Update]
Try this link to help generate your own race KML. It's still a work in progress, but it makes generating your own KML a lot simpler.
Use the map to select different checkpoints and write their numbers in the text field below the map separated by commas (,). When complete, hit the generate button and see the resulting race KML in the text area under that. To check it out, copy it and paste it into the Write KML field of the Map KML Dialog and hit apply.