I just received an article from Dr. Fernandez (of the NPF and the University of Florida) on this very issue.

It's entitled "When Should Levodopa Therapy be Initiated in Patients with Parkinson's Disease?" by Irene A.C. Halkias, Ihtsham Haq, Zhigao Huang, and Hubert H. Fernandez. Drugs: Aging 2007 24(4), 261-273.

It's a new article, so I'm not sure if it's actually been published yet, or is coming soon. It is an interesting read.

Hope this helps...