This might actually belong in the Etc category...
It can be an emotional topic for some guys. There are guys who would never give it up, and those who say never do it. Nearly all reliable data I've seen points to it being very safe. There is, however a statistical probability that it could occur. Some would argue that it's more likely to get hit by a bus than to get HIV through swallowing cum. One guy I've spoken to is extremely promiscuous with anonymous partners. He always swallows, and he says he's aware that there have probably been many men he's swallowed who are positive, but he remains negative. He's actually more afraid of syphilis and gonorrhea.
There was a Spanish study donein 2002 with couples in which one was positive and the other negative. No transmissions happened even though a high percent swallowed the positive partner's semen.
Swallowing
Additionally, there are levels of risk. If you go into the healthcare field, there's a risk of contracting HIV through needlesticks. It's all relative. The risks, in terms of one sexual activity versus another, have been estimated:
Risk of seroconversion
While some studies indicate there to be 'virtually' no risk of HIV infection under normal conditions, there are individual instances where it does happen; gum disease and poor oral hygiene can significantly increase the risks.
There was also a recent report (sorry, can't find the link) that re-confirmed saliva's ability to actually destroy HIV on contact. They were using this approach to develop spermicidal-type approaches to protecting people during vaginal and anal intercourse.
All that said, it's a personal decision that only you can make based on being informed. Hope this helps.