Technically I know how to implement Captchas, but I had a look at the
Pattern Library to see if I could find some "best practice" advice on
how to present Captchas to users, but didn't find anything.

As people are encountering these things more and more it makes sense
to make the look and operation from a user's perspective as uniform as
possible (obviously the technical details will need to keep evolving
to stay head of internet vermin).

I'm no expert in this area, so I don't really have anything to
contribute, other than to say I'd appreciate some nuggets of wisdom
from people who are familiar with the technique.

Joe.