"As a maintenance agent for opioid dependency, buprenorphine offers advantages such as a lower level of dependence and minimal withdrawal symptoms, due to its partial agonist properties at the micro-opioid receptor. Previous studies have shown 8 mg sublingual buprenorphine to be equivalent to 60 mg oral methadone in terms of retention rate and opioid-negative urine levels. "
I think the partial agonist property of buprenorphine can put a client into bad withdrawel symptoms, and is why they wait until they are on a much lower dose of methadone before starting Subutex.
I found this on the net. Try this link as well: -
http://opioids.com/buprenorphine/bupvmeth.html
Hope it helps