This is an interesting idea that I haven't seen before. Why shouldn't Twitter target trends at me? They know my tweets, and tweets of the people I follow. They could target trends, and ads for that matter, that I'd actually like to see.
Maybe they can't. They sure lack some core competence by not being able to block obvious spam accounts (Newly created accounts that: posts trend spam, do mass following and mass @mention, etc).
(ie. Look, this guy follows Cisco's Datacenter Twitter account... let's show him stuff from Juniper's datacenter team, etc)
Methinks the first rule of running general advertising is to run advertising. Otherwise you are injecting noise for no reason. At least advertising is defensible noise ("hey, you want your free client, you gotta pay the piper") and it has a fighting chance of not actually being noise (some baseball fans do buy lingerie).
Then I think they completely missed the boat.
The whole conceit of twitter is _you decide_ who to follow. They don't even start you out with a Myspace-esque Tom. Every user's timeline is unique.
So, if I'm following Roger Ebert, Kevin Smith, and Netflix, don't you think I would be receptive to DVD advertisements? Similarly, if I'm following Pogue, Mossberg, and Gruber, wouldn't I be receptive to iPhone app ads?