Charles Arthur's wondering why spam came through his CAPTCHA system, and concludes that people are probably being paid to sit there and fill out CAPTCHAs.
There are a couple of other possibilities. The first is that the CAPTCHA system he's using might be compromised. Some OCR systems can be surprisingly effective on them.
The second is his CAPTCHAs are being reproduced on another site for humans to solve. The canonical example would be where a visitor to a porn site is shown a CAPTCHA and asked to solve it before they can, er, continue. Unbeknownst to them, however, the CAPTCHA is actually coming from Charles' system, and the solution is then used to send him spam. This is "CAPTCHA farming".
Searching for "CAPTCHA porn" turns up a number of stories about this over the past few years.