Aha! Been all over the 'net, but I found this article from the excellent POZ magazine to be particularly enlightening. According to this Swiss study, if you're HIV positive and on meds and your viral load has been under forty for more than six months, than you're not going to transmit the virus when you have sex.
To be sure, there are plenty of nay-sayers out there who point out that it's impossible to prove a negative. However, I would respond by pointing out that there's absolutely no reason whatsoever that prevents the female Anaphales Mosquito from transmitting HIV the way she so effectively does with malaria. The only thing that epidemiologists can point to in order to show it doesn't happen is that... well... it doesn't happen.
Nay-sayers continue by playing their trump card: but who among us would be willing to take that risk? And there in the back of the auditorium would be me with my hand in the air. It's seemed obvious to me for years that it is less risky to have sex with someone who is HIV positive than someone who believes himself to be negative. Especially if the poz guy has been living with the virus for a while, he's probably on meds, and the gold standard these days is an undetectable viral load. But the neg guy may have sero-converted since he tested last, and shortly after sero-conversion his body is swarming with the virus and he's at a point where he's most able to transmit the virus.
Coupled with the success that the San Francisco Department of Health has had with encouraging sero-sorting (poz guys only having sex with poz guys and neg guys only having sex with neg guys), we arrive at an interesting logical endpoint: all things being equal, the safest possible sex would be what goes down with two poz guys; second to that would be a poz guy and a neg guy, but the riskiest sex would be between two neg guys. (Take that all you people who put "UB2" in your profile!)
That said, there is a down side. Recently diagnosed HIV positive men already face enormous pressure to start on meds sooner, even though they feel healthy and strong, and even though starting meds potentially means dealing with all of those hellish side effects like explosive diarrhea and neuropathy and such, not to mention having to worry for the rest of your days about your healthcare coverage. And with this news, the pressure will be even greater.
One sour note though. The swiss study only looked at sero-discordant heterosexual couples. Natch. The public health establishment closed the book on male-to-male transmission in 1986 when they conceived the mantra "Use a Condom Every Time," refusing to consider that for many people, this formula is a tad unworkable. But, they seem to feel that this lets them off the hook of doing any research that will provide gay men with information they need to make sound decisions.
For example, did you know that you're supposed to use a condom when you give or get a blow job? It's true! Do you know anybody who does that? And can you get HIV that way? In fact you can. But most of us have used the mosquito-logic mentioned above to conclude that even though there might be a risk, the risk there is negligible. Wouldn't it be kinda nice if someone with a research budget looked into this and other questions just a little bit? Nah. Let them use condoms.
Ah well, Science marches on.