Sunscreen does block UVB rays, and that's what's needed for vitamin D production. The better the sunscreen, the less vitamin D you can produce.
One thing that matters a lot is where you are. If you live in the southern US or anywhere similarly sunny, you're going to get enough sun through your sunscreen or through other small exposure areas. It you're in Canada, like I am, then you get less sun to begin with and it's a lot more important to get a few minutes of sun a day.
Sunblock is a good thing - especially for young kids and babies who can't or won't wear protective clothing and for people that are out in the sun for an extended period of time. Sun is also a good thing - we *need* it. But it's finding the happy medium of no sun/too much sun that's tricky. It's different for different skin types and different geographical areas.
Nevertheless, it's going to be sunny and 12 degrees out (50 something F) on the weekend and after this long crappy winter, I'm going to go out and enjoy the first nice sunny day!!! I might even take my hat off
BP