In reply to Damo:
I *think* I recall meeting some people in Almaty (last year) who said it was possible to drive in China, but it was necessary to be accompanied by a registered tour guide... I could totally be making that up!
I've heard the Baku/Turkmenbashi ferry can be a pain having to wait for a few days and then also needing to get through the country on a tight time frame (5-7 days I think?). If you haven't already, try speaking with some of the Mongol Rally crowd as it's a common route to go.
Uzbekistan might be a pain in the ass for registration and they're pretty tight with monitoring where you stay (at least in my experience) - Uzbekistan seemed more troublesome that way than Tajikistan or Kazakhstan.
Maybe it's changed since last year but if you want to cross the Tajik/Uzbek border the Penjikent crossing might still be closed, meaning the Tursunzoda crossing towards Dushanbe is your main option (head for the Pamirs!). The Shirkent National Park is worth checking out and it's kind of off the radar too.
Whatever you do I'm sure you'll have fun!