A few developers have already built their own maps into apps. OS 3.0, however, should make it a lot more common. Here's the definitive list of all apps that use fully interactive maps.

MobileNavigator North America

Price: $79.99

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MobileNavigator North America

By NAVIGON AG

The biggest name GPS navigation company now has a north American iPhone app. the $70 price tag will likely go up after August 15. Make sure to take a look at the free version of the app which will give you an idea of what sort of functionality is available in the full version.
G-Map CA

Price: $19.99

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G-Map CA

By XRoad Co., Ltd.

This is a more reasonably priced turn-by-turn app with voice guidance. It comes in several versions differentiated by geographic location. This one in particular is just California, but there's G-Map for the Western U.S. for $34.99 and a G-Map for the Eastern U.S at the same price. There's are also separate apps that cover other, smaller, areas.
AT&T Navigator: GPS navigation (subscription fee appears on AT&T bill)

Price: Free

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AT&T Navigator: GPS navigation (subscription fee appears on AT&T bill)

By AT&T Inc.

Offered for free, AT&T bills you $9.99/month for this turn by turn application on your monthly statement. Deleting the app does not cancel service - you have to go online or call.
Mobile Maps North America

Price: $59.99

Ituneslink

Mobile Maps North America

By Sygic

Another voice-guided GPS app. This one comes in at a hefty 1.62 GB with maps from Tele Atlas for all of the U.S.
Gokivo GPS Navigator - turn-by-turn voice guidance for 30 days

Price: Free

Ituneslink

Gokivo GPS Navigator - turn-by-turn voice guidance for 30 days

By Networks In Motion, Inc.

The first app to take advantage of Apple's In App Purchases sells for only 99 cents but is subscription based at $9.99/month.
MobileNavigator North America LITE

Price: Free

Ituneslink

MobileNavigator North America LITE

By NAVIGON AG

A free version of the North American version of Navigon MobileNavigator. It's meant for you to test out the functionality of this very expensive iPhone app for free. It still includes all the maps and is more than 1 GB in size.
MobileNavigator Europe

Price: $119.99

Ituneslink

MobileNavigator Europe

By NAVIGON AG

The European flavor of Navigon, a well established GPS directions company. Though the app is 1.65 GB.
Roadee

Price: $1.99

Ituneslink

Roadee

By aemade

This is one to keep an eye on should it get some great updates. It uses map information from www.openstreemap.org (the wikipedia of maps) and it's possible it could become as useful as wikipedia over time.
MapQuest 4 Mobile

Price: Free

Ituneslink

MapQuest 4 Mobile

By AOL

A fully-featured competitor for Apple's built-in Maps app. There's a one tap carousel of common locations one might search for, like hotels, gas stations or restaurants as well as the ability to specify multi-stop routes.
AroundMe

Price: Free

Ituneslink

AroundMe

By Tweakersoft

AroundMe has always been a decent alternative to the Maps app for finding places nearby, but it always lacked maps. Not anymore. Fully interactive Google maps are now built-in.
Mobile Maps

Price: Free

Ituneslink

Mobile Maps

By ivo3

Maps of South Africa. Not much more to say.

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