Garmin and TomTom may need to guard their backs in the portable navigation device market. Americans are increasingly using their cell phones for navigation, according to data from comScore, which says that navigation on the cell phone as of the end of May was up 82 percent over the same period a year ago. That’s a huge rise, but still a relatively small audience of users, which means the 7.6 million-person increase in cell-phone map users had a huge percentage impact.

However, more GPS chips in cell phones such as the 3G iPhone and the Samsung Instinct, as well as unlimited data plans that make access to navigation free (Verizon, c’mon you still want to charge me $9.99 a month for this?), means consumers are likely to turn to their existing gadgets rather than shelling out $236 (on average) for a single-focused device. On the other hand, prices for said devices have dropped by a third since last year, according to NPD research released yesterday. No wonder Garmin is getting into the cell-phone business.

chart courtesy of comScore

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