I was contemplating the future today [...I know, very dangerous], and tried Google Trends to capture historical global Web Search trends for personal navigation devices and map data, limiting my search keywords to Garmin, TomTom, Magellan, Mio, NAVTEQ, and Tele Atlas.
General deviations include traffic spikes during the holiday purchasing season, plus two notable anomalies for TomTom. The first coincides with the release of Google Earth and Maps in the summer of 2005 [most agree this time marked the beginning of explosive mainstream awareness in mapping]. The second arrives a month ago when TomTom announced their intentions to acquire Tele Atlas [reflected equally in the NAVTEQ-Tele Atlas comparison chart]. Other interesting trends include Magellan's relative flat traffic [which could indicate a well established brand], and Mio's domination in the news pointing to their aggressive marketing. One last observation... I conducted the analysis for all countries served by Google. When limiting the analysis to the US, Garmin inched above TomTom, while the NAVTEQ-Tele Atlas numbers remained steady.