IR rangefinders like the SHARP GP series (see
www.robotshop.ca) make great proximity sensors and are 100% legal.
Or did you have something else in mind?
Keep in mind that unless the rule book expressly forbids something, it is generally acceptable so long as it doesn't violate over-riding concerns such as safety and fair play.
Does your design violate the rule book? The only way to know for sure is to read it ALL, carefully. But generally sensors are pretty safe, so long as they don't involve lasers.
Just remember to read the section on custom circuits if you're planning on anything unusual.
Jason