I use my phone its just as good as any dedicated Sat-Nav. I have a Nokia 5800 and sat nav is free, no wires no nothing. Its the way to go Peachy 
Now that we will have to disagree on.
First, nothing touches a proper, built in satnav system. Even a really old CARiN as fitted to 1998 will outperform any portable unit for locational accuracy.
But forgetting built in systems, most dedicated units will outperfrom most phone based units - most dedicated units will use SSIII receivers or better, unlike the poor receivers fitted to phones (which need AGPS to get them started!). Also, screen size and volume can be an issue for most phones. Nokia in particular also have poor software, and non touchscreen models are too fiddly.
Peachy - whilst your idea is sound to hide wiring, be aware that scumbags will reconise the telltale 'circles' on the screen, and will smash a window to check in glovebox to see if its there. Even if you mount it properly, they will reconise the mount, and again go for the glovebox...