Good programmers check, Great ones test as well

I wrote a comment on Janet Gregorys post about Programmers as testers, and still feel I have to elaborate my comment a little more with the concept of testing vs checking and how these differ. While Janet already cleared out in a more recent comment that we are talking about two different things, I think it is of great importance to have these differences in mind when talking about testing, especially in the Agile development context where the word testing has become a word with many different meanings in various discussions.