Hi,
I took my 4 month old to Africa in the middle of their summer, and my ped said it was ok to put sunscreen on - that it was better to risk that then the baby being sunburned or getting sun damage. She recommended the no-tears Banana Boat brand. I was careful to not have him in the sun too much though, and so didn't put it on every day or anything like that.
Wrt the 6 months rule, she said it's because there have only been tests done on babies older than 6 months, not younger.
However, I am feeling more aware of the nonsense that goes into sunscreens in general - check out www.cosmeticsdatabase.com for their recent study on sunscreens, and their recommendations for safe ones which work, and there are ones for babies there too.
Another very good alternative is to put your baby in lightweight long-sleeved wear, or those uv-protective t-shirts/onesies. That's what all the kiddies wear in South Africa at the beach or by the pool!