But you will find, once you do get a smart phone, that it can do everything, and I mean everything a standalone unit can, including being mounted on the dash.
Others have stated that they have to pay extra for GPS functionality in their phones. Perhaps that’s the way it is for an iPhone, or perhaps the carrier, but for my Droid X on Verizon there is no extra charge for navigation.
After 6 months of constant use (I click my phone into the dash-mounted dock whenever I get into the car) I find that it is the most accurate GPS device I’ve ever used. I’ve never gotten bad directions. My wife’s old Mio is pretty good. Android Navigation is very good.
The other negative with my wife’s old Mio unit is we invariably had to wait at least a minute for it to “lock-on” to a satellite signal. It’s instantaneous, or it’s as close to instantaneous as to be a non-issue, on my Droid X.
You may believe now, as I did before getting my Droid X and my wife’s Incredible, that you’d still keep and use a separate GPS unit, but once you have a multifunction device that does all the things the dedicated devices do, and better, it’ll be difficult not to become ONE OF US. 
Seriously, my wife’s Incredible is a better music player/manager than her iPod Touch, a better GPS than her Mio, a better addressbook than her old PDA, a better phone than her old non-smart phone, a better camera than her old Canon Digital, a better video camera than her old JVC Camcorder, a better email client than, oh wait, she never had mobile email before, a better internet browsing experience than, oh, that’s right, she could never browse the web on the go before. With the included ability to use voice commands, and voice to text for emails and text messaging, it couldn’t have been named more appropriately as it is freaking Incredible. 
Like many of you, I was a standalone GPS guy at one time also, but they simply don’t hold up, in my experience, to the convenience of having one integrated into a smart phone.