Theme mobile optimization and upgrade suggestions
Mobile developing for D6 shouldn't be any harder than for D7.
Actually, i remember that when i was building my last responsive website, there were a few useful modules only available for D6.

D8 much probably won't be out before 2014, and after the official release, it's going to take many months before all the themes and modules will get ported... many people may not be able to use D8 until 2015 or so. Probably.

It will be indeed much more mobile friendly, so it might be well worth the effort of porting.

But I don't really see the need to port to D7 at the moment.

Whether to make a separate site on a subdomain or a single responsive site is a very subjective question. Anyway, mobile_tools will probably have everything you need for both scenarios: https://drupal.org/project/mobile_tools