The answer appears to be yes. I talked with Johan-Till Broer, who represents Garmin and its subsidiary Navigon, to see if the brouhaha over Maps -- which, with its new turn-by-turn and voice navigation features, was seen by some developers as a threat -- has actually boosted attention and uptake of these apps.
"We definitely saw an increase in sales for our navigation apps over the last couple of weeks due to the recent interest in map and navigation apps for the iPhone. It should be noted, though, that App Store dynamics are very complex and it's always difficult to reduce a spike in sales to a single factor. The fact that we were one of the first navigation apps to integrate with Apple Maps and provide public transit directions, the brand awareness that Garmin has as the global leader in GPS navigation as well as the fact that Apple suggested our apps as map alternatives in the App Store also played a significant role."
Garmin and Navigon get, I think, a significant boost because those solutions include Google Local Search and Google Street View, both of which left the bundled Maps app when Apple moved away from Google as a data provider for mapping. Both are still available as standalone free apps, by the way, and Google has now integrated Street View into its web-based maps for mobile.
[Just a reminder that the pre-iOS 6 Maps app was also developed by Apple, not by Google; while it's been informally referred to as "Google Maps" since the map data was provided by Google, technically the app itself was always "Apple Maps." We'll try to use "iOS 6 Maps" and "pre-iOS 6 Maps" for clarity, and reserve "Google Maps" to refer to the current web-based solution. –Ed.]
Broer also points out that there were other free third-party navigation apps on the iPhone before Apple made its move, but paid maps have always had significant sales.
Even on Android, where most phones work with Google's free turn-by-turn navigation option, paid apps still sell, including CoPilot Live, Magellan and both Navigon and Garmin. One of the main reasons is that many of these apps have all the map data onboard, and are not dependent on a data connection. Both the iOS 6 Maps app and the Android navigation app require data connections to function, though there is some built-in caching; iOS 6's app in particular seems pretty good about allowing navigation based on pre-loaded data.
For many users, Apple's Maps or some of the free solutions like Mapquest or Waze will be enough. For those wanting a premium experience, and are willing to pay, it looks like third-party navigation apps are here to stay. Apple's Maps "upgrades" haven't hurt the paid app suppliers at all, and in fact, seem to be driving sales of alternative solutions.