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.
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'm guessing there's a layer in between that clears it up for Twitter's own tools, and the third-party tools are left to clean up the stream themselves.
(On the other hand, Tweetbot has no in-stream ads, which mean both of these things are happening in the same "post-processing" layer.)
http://twitter.com/Yahairayqlcd http://twitter.com/Kenyetta992 http://twitter.com/Jade_482 http://twitter.com/Mozella_nxi http://twitter.com/Alane508
It's becoming a really big problem, every day I come across accounts that have bought followers\* (a few belonging to HN users) and it's... disappointing. Inevitable, but disappointing. I started putting together a website for checking if someone had bought their followers but haven't found the desire to finish it yet, but there are so many that do it, anywhere from purchasing 4,000 to 80,000 followers which can be done for ~$300.
I've been reporting the people doing this to Twitter but nothing has been done about it, ultimately it looks good for Twitter metrics so I doubt they care that there are people with 80k fake followers, that's an extra 80k users for their investors to salivate over.
As a side note, it's fun to pick out a random account (like those listed above) and see who they're following, there are some people you'd not expect to be purchasing followers that are. An alternate theory is that Twitter is responsible (after all, how can they not detect these obvious bots?) but I can't see why... well I can, but I don't think they would.
*I'm probably alone in this but people that buy followers really irk me because I like numbers to be accurate. The site I started making had a directory of people that bought followers, I'm wondering if I should finish it up, I figure it would be interesting for people to see that a large number of "media personalities" are just buying up their followers.