I am also having this problem.
See
www.currency-news.com/component/comprofiler/registers
My system is completely up to date i.e.
Joomla 1.5.17
CB 1.2.2
Captcha plugin 2.2.2
Server Debian Lenny completely upgraded today
I am experiencing the same problem as previous posters, i.e. the letters are missing from the Captcha image.
FYI, if I reload the Captcha image using e.g. the reload image function in the Opera browser, then new images are presented with letters included, but the first image is blank.
This is causing me difficulties with an important commercial client and I would be grateful if I can be given a fix asap.
Thanks you.