(As per your request)
I believe Joel stated that this was a suggestion by an investor - to wait and see about how best to monetize it. And I think it is pretty clear that none of the participants would say that model works in the general case.
I think it is a special case. Jeff and Joel have shown that they can execute and provide value. This is not just a shot in the dark. (I happen to think the way they are implementing the new Q&A sites is somewhat broken and have stated so on some of their web postings but that is not relevant here)
Regardless, the "Wait and see" attitude for revenue is pretty reasonable given that they already have a site that went from 0 to 6 million uniques per month in the period of 18 months. The VCs probably have a BETTER understanding of how to do revenue than Joel and Jeff, but in any case they figure they will get the people, behavior and interest first, then see how best to generate revenue.
I don't happen to particularly like the business model of "get people to visit my site and then we'll pay for it later" but it apparently works sometimes. In this case it probably will as well - there are a lot of bright and connected people behind it.
Even if it fails - it was only 6M, and I suspect it will only take perhaps 2/3 of that to figure out if it will become profitable, but I really have nothing to base that on - it is just a hunch.
RE you comment above:
VCs are looking for the mega hit and
saying "let's not build a revenue
model, let's just get a user base and
figure it out later" just seems
I think the VCs in this case are taking a harder look at the PEOPLE involved rather than any particular business. It is a symbiotic relationship - Joel, Jeff, et al get the benefit of the funding as well as the breadth of experience the investors provide and the investors get a team that has already proven itself a few times over.
There is also possibly just the cachet of being involved in the Jeff and Joel show and being part of the SO company.