Every one of the initial ST2s I saw had the nav (along with the moonroof), though they're both optional, and unrelated as far as packages go. Moonroof is a standalone option; the nav is bundled with stereo/MFT upgrades.
Why do you say this is the last car in which you'll have nav?
I've tried driving using my smartphone as nav, and I'd much rather have a big screen mounted in the dash. Much as I like Google Maps, it's not a replacement. At the least, you need a spot to mount the smartphone / dedicated GPS on the dash where you can see it, it won't slide around. Hands-free operation is a bonus.
Also, I like a dedicated nav system for having a local copy of maps. Google eventually let you pre-download information about places you intended to go, but the smartphone is pretty crippled without an active, quality, data connection. That's not always available, and you don't want nav to depend on it. The flip side is having a decent -- and inexpensive -- way to update that local cache.
The SD card heads in the right idea. Now the car just needs to hop on the WiFi and download free updates while I'm asleep :)