“Starting later this week we’ll be limiting those on the whitelist to 20,000 requests per hour. Yes, you read that right: twenty THOUSAND requests per hour. According to our logs, this accounts for all but the very largest consumers of our API. This is essentially a preventative measure to ensure that no one API client, even a whitelisted account or IP, can consume an inordinate amount of our resoures.”
I haven't built an application on the Twitter API. But I know that twenty thousand requests per hour isn't that much, really, especially if you have big plans for your application.
The folks at SocialToo think so too, and explain why in a lengthy post (also check out the interesting discussion in the comments).
Once again, it boils down to the old discussion: is it too dangerous to build an application on an API you can't control? Whether it's Twitter or Facebook or Digg or any other web service that provides a public API, creators of third party applications are always at risk that their efforts will simply be erased by some unpredictable move on the part of the company that controls the API.