Well, just thought I would put my experiences up on Sponsored Tweets since I'm likely more representative of the majority (ie I don't make 5-6 digits USD a month from Internet marketing, and I really only have used twitter to get some traffic for articles on differet sites I have).
Anyway, I was reading shoe money a while back, and signed up for Sponsored Tweets, http://bit.ly/3pyQZg
with my basketball twitter account. (just under 1800 followers now).
I listed my account as open for advertisements under the "Entertainment" ad category and I set my list price to about 20% less than Sponsored Tweets recommends (the advertiser gets charged double this amount).
Its resulted in $20 in the past 4 weeks for me. Several political ads for the New York mayors race, K-Mart, and Volvo have been the tweet purchasers. Total of 9 sponsored tweets sent three times if the CTR of 3 clicks is not met.
I have not mucked with the other end of "buying" tweets yet, but seems interesting to me to see if the conversion rate of a targeted twitter following exceeds that or is less than that of a similar Adwords one...
If you read everything on NB, I yabbed a bit here
on some other twitter tools and the same program a bit, just not as much info.