Disclaimer: I am not a doctor and this is not medical advice, yada, yada, yada.
That said I remember searching the net for research about this a few years back. I only found one study, which took place in Portugal or Spain, i don't recall which.
The study was made of several hundred married couples. In every case, one partner of the couple was HIV positive. These couples had made the decision to continue with unprotected oral (both fellatio and cunnilingus) even though one partner was infected.
Over the course of this study, a total of some 27,000 acts of unprotected oral sex took place. At the end of the study, none of the uninfected partners tested positive for HIV. That's right, zero transmission.
This result seems very reassuring to me.
I was not able to find any other studies like this. Also, I may not have the numbers exactly right, so by all means google it to see for yourself.
Cheers and good luck.