What do you mean print selection? Do you mean printing selected text? Yeah, I don't see how to get that to happen on Safari. It shows up as an automatic option if text is selected in the print menu when I'm printing from Word...or pretty much anything else. Let me go look if it's specific to Safari, or any browser.
While I'm looking, an alternative (though cumbersome and probably not what you really want to have to do if there's an easier way) is to use the application grab to take a screenshot (you can select part or all) and print the screenshot.
Edit: Huhm. It seems it doesn't matter what browser I use (I tried Safari, Opera and IE), you can't print a selection in any of them. I never noticed that before. That's a bit annoying. I don't see anything in help on that topic either.