I was looking at the fact that wikipedia's discussion page always have a : in the url, eg http://en.wikipedia.org/wiki/Talk:Search_engine_optimization, and was wondering if this was the reason behind none of the talk pages being indexed. Do spiders find 'non-standard' URL characters like : and (possibly?) + (never seen it in a link though) impassible hurdles when trying to index pages?
Google will treat it as a space in most instances. In the Title of a page it actually comes quite often in google results. Just google Wikipedia:talk!