I'm trying this setup on my wife's SV when her Evo. In setting this up, I have discovered that there are different kinds of screens. Some are pressure-sensitive and work great through the membrane. Some sense electrical discharge, like the Evo and are iffy at best. THe iPhone is like this ... dunno what the Droid X is. The thing is, the membrane itself is sort of rubbery and doesn't easily allow slidding, especially with gloves. The Google Nav relies on pinching/expanding. This is OK for our use, as she just wants to listen to podcasts and be able to pause it (i.e. a one-button deal).
Another thing is that, while it works, the Google Nav app is sort of crummy. It doesn't have a pleasant voice, volume control is tricky, it's not easy to navigate the menus with gloves on, requires a cell signal to work, can't pre-create routes, use multiple waypoints, etc. I use it when I am traveling for work (in a car), but for recreational use the Zumo is unbeatable. It does one thing very well (and the crappiness of the MP3 player is tolerable).
There is also the issue of battery life. You will NOT be able to run Google Nav with GPS, bluetooth, and data connections more than a couple hours, so you'll need to rig up a way to charge the phone. I don't know about the rest of you, but I've have a b*tch of a time figuring out how to charge my Epic on the bike. There's something about the FJR's electrical signal that barely puts a charge into the phone.
If there's one thing I have learned in my short 38 years, it is that you get what you pay for. If you can afford it, get the Zumo and avoid all the crappiness that comes with trying to make the wrong tool work. Consider all the things you will need (aquabox, hard wiring, touchscreen issues, volume control, etc) vs. a polished solution and in my opinion, it's worth a couple hundred more dollars for the Zumo.