This works fine.
I have an ipod Touch and I just go to Google maps, put my route in and let it find it all whilst connected to WiFi.
Then when I am out and about, all the information is still cached. It seems the only way for it to get rid of this information is to ask it to do another direction search, but up until you do that, it will simply display the info for as long as you need.
I recently used the function to get from Bristol to a friends house in London by car and it worked brilliantly. One other cool thing you can do is zoom into specific ares you want to see more detail in and let it load at all the levels, and it will also retain this information too! I have even went along an entire route and let it lad all the map and picture data and it retained it all.
It would be great is google maps on the touch and phone could have a little option to save the route for when your not connected, because, whilst it works right now, it's hard to know when you are going to start writing over the cached info or if it's limitless.
Mel