Baglux now do a little tank bag to hold a GPS unit. It's split into 2 parts, the GPS lives on top, and there's plenty of space underneath for a phone and iPod, plus slots for cards and documentation should you so wish.
It's also got a little wiring port, I've got both GPS power and home-made Autocom loom running through it so I can just drop it on the bike, snap in the 2 connectors, and away you go. I'm just trying to work out whether to use the power connector on the headstock or get secondary AUX power on the beak.
They also do a magnetic version if you don't have/want a Baglux tank cover.