In SharePoint 2007 if you created a custom User Property and set it to be indexed, it would automatically be used in People searches. This is not the case in SharePoint 2010.
To use a new custom User Property in search in SharePoint 2010 you must at a minimum add it to the "ContentsHidden" Metadata Property.
First run a crawl to put the new property into the index.
In Search go to Metadata Properties
Find and Edit the ContentsHidden property.
Click on Add Mapping.
Filter for the "People" category and find your new property
You may need to run another crawl, but you can now find people using this new property