I think the key issue here is about "Viral Load". That is the amount of HIV found in the body. As you'll know, the new drugs, which I've taken since 1996, reduce the amount of virus to what are called "undetectable" levels - when they work well! That doesn't mean you have no virus but it does mean it's low in your body.
But when people get first infected with HIV their viral load skyrockets. A friend who very recently became positive (he knows because of symptoms he experienced) had an original viral load of 600,000 (that's high!). Now three months later, it's down to 20,000... and probably will get down even lower as his body's own immune system responds to the HIV. That's absolutely textbook reaction. BUT before he tested for HIV, in the few weeks after infection, my friend carried on having sex with people including oral sex. I believe it's at that time when infections happen because your viral load is very high. Incidentally, your viral load is also high when you get very ill (but usually you're off sex by then! I know!).
Here in London, some of the discussion is about trying to reach people about safer sex info when they are first infected because that is when they themselves are most infectious to others.
In other words, you have passive sex (oral or anal) with someone who you later find out is positive, you go to the clinic, they say "Be really careful about what sex you have for the next two months and watch for symptoms of "seroconversion" (that's when you go from negative to positive). The symptoms are night sweats, flu like illness (without phlegm etc), rashes, tiredness and fatigue etc.
It means if you have become positive, you're not putting others at similar risk plus, as the first correspondent notes, you can get treatment at this stage to "blow" the virus out of the body (that's the theory though I'm not sure it works).
I'm not sure if undetectable viral load means you're much less infectious, but my gut feeling on this, having talked to doctors, is that you are. You're certainly much less infectious than someone with a very high viral load in terms of risk and my real sense is that new infections are coming from people who themselves don't know they've just been infected. It would explain alot, because I'm not convinced there are loads of people who know they are positive going around having unsafe sex. People aren't that stupid!