I am having this exact problem. I copied this from a Google newsposting.
Anyone have an answer?

I have followed all the directions in the SharePoint help files to link an
Exchange Public folder to a document library.

I went into SharePoint administration then manage virtual sites then my site
then site general settings. I then entered in my exchange server address
there (http://exchange/public), set it to update every 5 minutes, and
clicked ok.

I then went to a document library and clicked advanced settings and entered
in the name of a public folder ("Foldername"). It listed the
http://exchange/public right next to it so it seemed ok and I clicked ok.

I waited 5 minutes to make sure and it still didn't do anything. No
documents appear in the document library even though there are around 50
e-mails with attachments in that public folder. What am I doing wrong? I
know if I go to http://exhcange/public/Foldername I get an OWA page with the
documents.