And you really think they're not going to use that piece of prime real-estate for text advertising? Wake up and smell the coffee :-)

I don't doubt that they are, but these are not ads.

HN has a lot of prime homepage real estate to the right, but if they put a link there to the submission form, it's hardly an ad :)

Yes, and so my point is, this is a masterful approach to pave the way for placing ads there.

The standard way is to place ads and juggle them about until you've figured out where they attract the more clicks. Twitter's way appears to first manufacture a high-interest area (by putting cool twitter stuff there), then, once people are used to seeing nifty stuff there, start selling that nifty space to others.

Actually, they are very real, internal ads. They rotate and point to different sections of the site, sometimes it's widgets, other times it's the search, etc.