The Twitter account I use for the website I work for which has 10,000 followers that are follows mainly through our website (we have the "follow us" button on a high trafficed page) are mainly from Twitter "lurkers" and I can't find a single one that matches the purchased followers pattern I mentioned, you're welcome to check yourself: http://twitter.com/i/#!/redstonewire/followers
On top of that I've checked with all the accounts I suspect of purchasing followers when they gained their followers: it's always within a few seconds that they've done from ~1,000 to 5,000, or 1,000 to 60,000. Here's an example of one account I was monitoring, you can see the jump from 2,000 to 4,000 took place at the same time: http://i.imgur.com/jUMNj.jpg and here's another that went from 0 to 60k: http://i.imgur.com/KwJZj.png
I don't mean these sort of accounts in isolation: 1 follower matching the pattern out of 100 is fine, but when a user has 10,000 followers and 9,900 match that pattern don't you think that's an indication of follower purchases?
Another type I didn't mention are the type that are legitimate active users that have authorised applications that are using this auth to generate follows on demand. These are interesting but not relevant because they're not created accounts just for followers, but legit accounts being used.
I am followed by users that match the pattern you're describing, but I assure you I have never bought them. It's not coincidence that the accounts are of attractive young women- they want reciprocal follows. I am almost always followed shortly after tweeting, so I would venture to guess that they are plugged into the Twitter API's "sample" feed, and sometimes I am unlucky enough to be included in that sample.