I do that too, already, 0 reply threads get noindex until 1st reply is posted, its a simple if conditional.
I do this mostly because a thread with no replys is not a good user experience and carries a high bounce rate imo.
But that doesnt cater for users posting one word threads then a second user doing the same.
A 2 word/2 post thread bypasses the noindex trap, as does someone posting a thread with the words 'Help' in the title, which is common.
These issues are less these days as i am wiser and edit bits and pieces, but that doesn't stop there being a bit of rot imo that was hiding in a dark corner of my site.
I bet most forum owners would be shocked if they run the same script of their site. Panda aside content like the above is a horrible user experience and wastes bandwidth with crawling, sever load waste, pagerank flow and loads of other stuff.
It's not like its getting search referrals to warrant it.