With the advent of small GPS units such as the GARMIN Edge and Forerunner, the potential for route analysis post racing is growing.

Should the use of GPS units, that don't contain any mapping data, be allowed under the MTBO Rules?

As long as maps don't contain grid and the coordinates of controls are not published, using a GPS helps nothing for an advanced competitor. It may give some information to the very beginner if the question is in which corner of the map he/she is in. GPS without map display is virtually useless in MTBO. Therefore I would not limit its usage in any way.