Target is deprecated in a Strict doctype, not only for causing some users of full-screen browsers the gouging of the eyes when they keep clicking their Back buttons and nothing happens (new window covers the old one entirely and they don't notice), but also because some consider it a "window behaviour" and therefore relegate this to Javascript. Which is possibly a better idea, coupled with PRG which gives you some added benefits anyway. Additionally many users block the action of target within their browsers; something to be aware of if you're relying on this behaviour for anything.

Originally target was meant for linking between frames in a frameset page. Of course, browsers really don't drop functionality, so it's not like the target attribute will ever stop working or anything... : )