Is there a bias in top tier PhD admissions at top tier universities to favor those with undergraduate degrees from other top tier universities?
Excuse my short answer: absolutely. 

Obviously, a school with a good, or even a great name, will give a candidate a big boost in his/her application. This is a rational decision: the recruiting committee wants to minimize any risks and they don't have much time to delve seriously into the history of each candidate. So if you graduated from a top school with excellent credentials you will have an edge over those with equal credentials but lesser perceived schools. 

Good references from colleagues of faculty in the department you're applying to, and good ties with your school are also strong factors for admission.