Security Warning

I don’t know if this has been posted before, but I’m starting to notice it more and more (especially in the last few days). Whenever I browse the Comodo forum, I get the following warning from time to time:

http://img233.imageshack.us/img233/3134/37529725.png

Why is this happening?

The way I understand it, this warning only pops up when unencrypted data is being transmitted on an encrypted web-site. What’s going on here?

The pages are encrypted, it is what members place in there signature etc. which is not.