In my current project all of the sudden the people search stopped working and the number of items in the scope were zero.
I did some research and found that to enable the people search you need to add the following url to the content source (if your portal url is http://www.demoportal.com)
then you need to add sps3://www.demoportal.com
check this post talk about Crawling SharePoint sites using the SPS3 protocol handler
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