I'm watching this product space too. I will not mind too much paying more for this solution than I would have for a stand alone gpsr for one important reason. Thanks to Apple, anyone providing an external gpsr to the iPod Touch have to provide the location information to all apps on the Touch through the built in location services. This means that not only do I get to use whatever app the gps cradle comes with, I also get to enjoy any other app I choose to buy. I can use Groundspeaks app, apps that specialize on hiking and show topo maps, location logging apps, etc. It's a GPS solution that has many different types of applications.
It is rather fragile compared to an outdoor gpsr, but I got used to that with the Garmin iQue.
So far, it looks like there are 4 ways to get an external gps for the Touch. To summarize:
www.posimotion.com sells a wifi gpsr that has been out for over a year. They have been promising the US navigation app to go with it "very soon" for the same long year. Sigh. Currently $100 for the gpsr, app should be around $40, total cost around $140. The gpsr has it's own battery and could be used for geocaching.
TomTom has a nice app/car cradle solution that is actually out and can be purchased! $99.95 for the cradle, currently $59.99 for the app. Total cost around $160. Cradle has no battery, not really intended for geocaching. No microphone on the cradle for the touch.
Dual has announced a gps cradle for the touch. Should be out any day now, for the past 3 months. Should be around $170 for the cradle, app is free and already in the store. Total cost around $170. The cradle has it's own built in battery, and so could be used for geocaching. The cradle also can charge the touch, but not when the gps is on. It also has a microphone, which could be useful.
Magellan has a car cradle that will work with the touch and its also out. $129.99 for the cradle, $79.99 for the app. Total cost around $210. Ouch. Cradle has no battery, not intended for geocaching. Cradle is the same for the iPhone and the Touch, so it has a microphone but it connects via bluetooth as its intended for hands free calling. Not sure how well that will work with the Touch.