I’ve been asked a few times in the past year about opening links in new windows. Just because a tag, property, or attribute is there — even valid — doesn’t mean it’s appropriate to use. For instance, did you know there’s a valid CSS property to make things blink a’la the infamous <blink> tag? Doesn’t mean that’s okay.
Anyway, I wrote a blog post for 10up on the topic in case you want to know what you should be doing.