Google is moving into your market.

For tech companies, few words are more frightening, and yesterday we saw why. The mere announcement of Google Maps Navigation sent shares of established GPS device makers like Garmin (GRMN) and TomTom into an ugly downward spiral.

Shares of Garmin lost 16 percent of their value and shares of TomTom plummeted more than 20 percent to a new 52-week low. This, despite the fact that Google’s (GOOG) new turn-by-turn mapping service is currently only available for Android 2.0 smart phones like the Droid from Motorola (MOT).

Clearly, investors already see where this is headed. Who will pay for a Garmin or TomTom unit when they’ve got an Internet-connected smart phone that offers turn-by-turn voice guidance for free? It might be a bit slow coming out of the gate given the current paucity of Android-based smart phones, but once more handsets arrive at market, Google Maps Navigation will undoubtedly have a significant impact on sales of dedicated GPS units.

And if Google brings the service to Apple’s (AAPL) iPhone, as the company hopes to, it’s going to be even worse.

“With this product, Google injects itself into a new market and has the potential to cause serious pain for the existing players, like Garmin and TomTom, while giving potential customers one more solid reason to switch to a Google-powered phone,” said Gabriel Consulting Group analyst Dan Olds. “It’s a lot easier to justify the switch to a new device if it takes the place of two devices, like a phone and a navigation device, and if it does the job better.”