Good idea. But how will this affect phones with memory card?..
I'm actually considering doing something with the space behind where the factory tape deck would go.
Someone here on the forum posted pictures of the inside of the OE CD stereo after removing it, and apparently if you don't have a tape deck, that's just a faceplate covering a tape-deck-sized cavity. If we remove that cavity, there should be room to insert a little fold-out-phone holder (similar to those pull to pop out cup holders I recall on other vehicles).
Currently in my other car (nissan Sentra), I have a pop open storage space on the top of my dash board, over the stereo. I bought a $5 plastic cell phone hip holster for my smartphone off of ebay, and instead of attaching it to my belt I attached it to a little bar that sits inside the storage bin there. When I want to use my phone for music/navigation, I open the "secret door" (as my wife calls it), and fold out the clip.
The trick is to find a belt holster that allows your phone to sit face out. So far, I've been able to do this for my last 4 smartphones (been doing this for years, before smartphones were considered "cool"), and every clip was some cheap $5 plastic. Perfect for the task at hand.
I'm hoping to do something similar on our new 5... I'm guessing the tape deck cavity is the most promising, but not sure how much work would be involved.