I can't find a source on this, but it says:

The rate of HIV transmission with sexual intercourse is much lower than with other sexually transmitted diseases-approximately 0.3% per sexual contact with an HIV-infected person. However, some persons have become HIV-infected after a single sexual contact, while other persons have remained uninfected after hundreds of contacts.

from: http://library.med.utah.edu/WebPath/TUTORI AL/AIDS/HIV.html

Looks like it is a med school in Utah?

In a University course we learned that the chances of transmission are certainly less than 100%, but as I can't remember the numbers off the top of my head, I will have to check my textbook when I get home. I don't want to give you inaccurate information.

Even if it is way less than 100% - why take the chance?

I agree with Anne. It's not a sure thing that you will get HIV from having sex with someone who has it. I don't know what your exact chances are, and you know what? Exact chances don't even matter here. You might get HIV from sleeping with someone one time, or you might not get it after sleeping with them a dozen times. So the important thing is not to take the risk in the first place. Always use protection. If you already have taken the risk, then wait at least six weeks and get tested.