I'm just posting this in case it can help someone else.
I've been using "really simple captcha" with "contact form 7" and WP installs for a long time and never had any problem with it until tonight when I set up a fresh WP for a friend.
Even though I had installed both plugins the same as I always do, and on the same server (even same account with other WP installs) , for some reason on the contact form in the captcha image location it was just displaying the word "captcha" instead of providing an actual unique captcha code.
I was baffled for a bit, and then I tried to think of anything I did differently. Came up with only one thing:
One this new install I had not yet gone into Dashboard > Settings > Permalinks and yet chosen a a permalink structure yet.
After chasing my tail for a while, it turned out that the solution was as simple as going into Dashboard > Settings > Permalinks and setting a permalink structure.
As soon as I did that, the Contact Form 7 / Really Simple Captcha started displaying actual captcha codes instead of just the word "captcha" in the box.
I hope that this will help someone else someday.
I'm very happy with the CF7 / RSC combination of plugins for a long time now, and this was just a little bump in the road. (And probably only coincidence that it only surfaced with WP 3.5).