I was asked this by a university in Arizona with thousands of public folders. I found a free tool call PfRep on our Codeplex site that will analyze and help clean up Exchange 2000/2003/2007 public folders via visual color coding (red is heavily used in last 10 days and various shades of yellow represent folder used in last 2 months, 4 months, 6 months, etc).
Grab the PfRep 1.4 tool here.
I also found a tool called to help you migrate your Public Folder to SharePoint. Grab it here. There are also 3rd party tools to help you migrate your public folders to SharePoint such as Quest Software and others.