SharePoint 2010 Search can search multiple location. One of those location/content source is Exchange Public Folders.
This article talks about steps that are required to search public folders in Exchange 2007 SP2 or later (which includes Exchange 2010).
Step1: To get the URL of Public Folder do the following:(assuming that you have already created public folder)
Step2: Go to Central Administration-> Click on Manage Service Application under Application Management.
Step3: Highlight Search Service Application in the list of Service Application. Then click manage.
Step4: Under Crawling, click on Crawl Rules. Click on New Crawl Rule.
Step5: Click on content source under crawling -> New Content Source. Give it a name and then choose Exchange Public Folders. In the start address paste the Public Folder URL.
Step6:Under Crawl Settings choose "Crawl everything that comes under the hostname".
Step7: Set the Crawl schedule according to your convenience. In the content source priority choose normal.
Step8: Under Start Full Crawl, check it.
Step9: click OK
And its done!!!...
Next time do you a search for content in public folder it will be visible in the search results. (If you have atleast READ permission on the public folder :) )