I'm against the idea of resorting to ads.
From what I can gather, Toady has been doing well enough from the donations alone, and ads will only discourage other users (that particularly hate ads in any place, especially forums) from signing up and even donating (also assuming the ads make the revenue anyway; or the stigma of having ads feels like betrayal on a forum; especially if the admin is collecting donations as well), there would be increased chances of malware/adware without Toady's consent, it can break the character of how the forums are, and there will be tons of topics about when Toady would be getting rid of the ads, and lastly, a great portion of people online have an ad-blocker program and other forms of virus/malware protection (as well as a stylish/greasmonkey script to eliminate ads or ad visibility) active to be a countermeasure anyway, which renders having ads up pointless anyhow.
How do I know? Because I do that. I have more countermeasures than I need to prevent ads from interfering with a site's enjoyability, as well as to prevent my computer from getting infected by some random ad from a random site, or pop-ups (pop-unders, or -minimized, -intangibles (they exist), javascript and action/flash script in-lines (in images even (rare, but possible nonetheless, and hard as hell to detect from any scanner)). I mean, I run virus scanners and Spybot/Malwarebytes every other day just to feel safe. That's how bad ads have gotten these days; where you actually fear your computer breaking down just because "Ford is built tough.".
In agreement with others, the lack of ads is actually one of the many reasons I like it here. Any other forum I've been to had ads every 5 posts down, and on the side bar, and on the side bar (opposite side), on the upper portion and the bottom portion of the thread (simultaneously even, including all of the above), and even set themselves up to look like a legit post. And in some cases, you can't read a certain post or go another page without clicking on an ad first AS A REQUIREMENT TO PROGRESS. It has happened before, and it was one of the many reasons I quit the places. Hell, it's another, among tons of, reasons I no longer use social networking sites (Facebook/Myspace/Google-whatever/wannabe etc.).
EDIT:
On a side note, I find it ironic that I have a, although freelancing, career where I make these things (not main focus, but an option). At least I have the choice whether I want to or not (ads, that is.). When designing, I want to work on things that are practical, not annoying. In other words, if I am going to make an ad, I choose to make one that is used in a print media, rather than digital (more room for creativity there anyhow). Or at least anything that's not related to the types of ads described earlier.