Some time in the near future, Twitter will begin sending out advertisements alongside of tweets. It's a revenue model which makes sense for them, and it would not require any drastic change in the service. They WILL do this because they need to come up with something sustainable, and quickly.

Not matter which way I think about it, it seems to always reduce to a adwords/adsense-type model. The adwords model will basically let advertisers bid on keywords. When a twitter user gets an update from their friends' timeline, a certain percentage of the tweets are ads which are contextually relevant to their followers' content. So if I'm following lots of sports icons, I'd probably get offers from sport merchandisers, etc..

Also, the adsense model will let high-ranking twitter users make money from their subscribers. A twitter user with two million followers is inherently more valuable to the service than one with 20. So twitter will probably allow them to turn on advertising, which will show advertisements that are relevant to the high-traffic users' content. A side-effect of this could be that the more celebrities you follow, the more advertisements you'll receive, unless the number of ads is limited.

Another side-effect of this is that twitter clients will have be able to have a revenue model. It would be easy for clients to just block ads, but with some incentives they could show ads by default.