I have to give a contrasting opinion here. A domain homepage with PR 8 distributes its linkjuice internally, and how much PR each internal page gets - for example, forum threads or blog articles with comments - is reflected by the PR score for that page. Thus, a page with PR n/a or zero shows up as such because the internal link structure of the domain has sent it hardly any link juice. It is true that the PR may not have updated yet, but I believe this applies in a relatively small number of cases.
A page has PR0 or PR n/a because it has negligible link juice going into it. If it's got next to nothing, then there is little benefit in tapping the page with your link. Relevancy/authority may be strong factors here, but It also must be admitted that the core of a link's importance comes from the amount of link juice given to it by the page rank flow system.
Because of this, I would answer that the individual thread is all important. Its PR is the main determinant of its value for link juice. However, I have to concede the possibility that you may get some bonus points from authority/relevancy as it is widely believed on the forum, although I have not personally seen any evidence for this. Google actually reported that .edu links are only seen as so powerful because they have a lot of aged, high PR links - not because of any magical authority effect.