If you are not able to see folders in Exchange 2007 install a tool called PFDAVAdmin to see if the Public folders are there. Did you replicate public folders from 2003? If so and they do not show up in Exchange 2007 us PFDAVAdmin to touch every folder in PF and that should initiate replications.
Take a look at this
http://msexchangeteam.com/archive/2007/10/30/447339.aspx