I think this is a bit of a naive post from dotcoma... Twitter haven't waited this long to just spam adverts at people randomly.
Twitter are smarter than that. I think they will be more Hyper-targeted adverts that will be way more subtle than annoying. That's what I would be thinking about doing.
I've had experience of putting what I'm trying to do on Twitter and having a company send me a message offering their service. I'm not sure if it was a bot or not.
It occurred to me that maybe in a few years time we won't be googling for stuff by rather just "asking twitter" and automated bots/crawlers will respond based on where I am, what I asked for and details from my profile.
If twitter could bake in some kind of spam prevention (or reputation system?) it might just work.
That is something that Facebook have been teasing Google with for years, that they could be a better search engine because results would come based on your friends results.
I think this is very realistic reaction, people listen to the people they trust more than someone on I'm feeling Lucky. I'm basing my own startup on exactly that ethos.