There have been blog posts discussing the huge amount of revenue that Wikipedia is missing out on by non running ads ($10s of Millions I presume), and what they could use such cash for (give it to charity?)

I reckon:

1) If you start out saying “there will never ever be any ads”, you need to stick to that, because that is the basis on which people helped the service

2) Google did it before with Google Directory (a copy of DMOZ with ads attached), and could quite easily do it with Wikipedia and out-rank Wikipedia in search queries

3) It is extremely important to consider immense success when creating your website - because it just might happen. Freecycle is having similar problems right now - a massive user-base but a vow to always be non-profit. This leaves you vulnerable to big business, without the funds to fight back.