Ditto on custom steel and Ti framesets. Value will depend on the reputation of the builder and design characteristics. Many of these builders go to NAHBS or similar regional shows.
How about the following?
* Chris King and/or Phil Wood hubs
* Crank Brothers products due to their awesome and unique American designs.
* Bikes and other products from the current fixie & mixte revival era I have the feeling will be highly collectible when those kids turn 50.