[ufo]

In my experience, a lot of the time when programmers say big O, they actually mean big Theta...

[adrianN]

Programmers need not be very good computer scientists. The actual scientists publishing papers on algorithms usually get it right.

[tsimionescu]

Yes, I agree with that. However, if f(n)=Theta(n) => f(n)=O(n), so they are usually not wrong, technically, just unnecessarily broad.