I've seen the thread prefix/tag thing done well and I've seen it done poorly. I'm afraid I can't find any examples off the top of my head. But I can say the current XenForo proposal looks like a good start. Perhaps you could design it so that it's a nice looking icon or subtle hint instead of the jarring, bright-backgrounded text seen in the pic/video (a smattering of differently-colored tags would kill the cleanness of the thread listing).
Considering it will let you do custom CSS that should be easy enough. One with a soft aluminum look for original iPhone threads, black plastic look for the 3G (glossy but not glassy, if that makes any sense), white plastic and shiny text for the 3GS, then a black glass look for the 4 and a glassy(er?) white look for the 4S. I think that'd look pretty snazzy.
If it is easy and clear for the users how to place one on a thread (might need to separate the "dropdown" into a set of radio buttons above the thread title), it'd work really well.
The single iPhone forum in any case would be hopping with activity, and make it more like a real-time chat in which people get answers more quickly.