I just pre-ordered the beta for Unity 3.0, and I was given a key. Is the key just for Unity 3, or is it for Unity 2.6 Pro also?
If you paid for Pro, you get both the current 2.6 and early access to Beta 3 when it comes out, so yes.
Pre-order Unity 3 - "The license purchased will be valid for both Unity 2.6 and all future releases of Unity 3.x."
You should have gotten a Download Link in your email, but here it is again.