OMZen - 8:14 am on Dec 18, 2010 (gmt 0)
Like someone said earlier, there are few established sponsored tweet networks out there which act as a platform for connecting tweeple with advertisers. Simple Google search for sponsored tweets can land you on some of those networks.
There is mandatory disclosure policy for such tweets with hashtags like #spon or #ad , etc.
The rates can vary from single digit to upwards of 4 digits for a single tweet.