You can still boost via an unlinked high level, but the old method of PLing via dropping group and killing does not work any longer.
Did Blizzard change it back? I found this post by a blue dated 8/28/08:

Let's say we have Account A with a level 70 character, Account B with a level 1 character and Account C with another level 1 character, all three characters are grouped. Account B and Account C are linked, will they receive triple experience?

  • A: Bonus experience should only be granted if the experience is not trivial to anyone in the group. Our Quality Assurance Team is looking into reports that some of these restrictions are not consistent and hope to resolve the issue soon.