Having configured SharePoint 2010 Search I had a problem whereby some users received zero results when searching SharePoint content.
In this configuration SharePoint was in one forest (A) and the users who got zero results were in another forest (B). Users in the A forest were able to search and get results. In order to resolve the issue I had to convert the SharePoint Search Service Application to store the SharePoint ACLs in Claims format.
To convert the SharePoint Search Service Application open the SharePoint Management Shell and run Get-SPServiceApplication. Copy the ID of the Search Service Application.
Now run the code below replacing %SearchServiceID% with your Search Service Application ID.
$SearchApp = Get-SPServiceApplication %SearchServiceID%
Run a full crawl and users in the other forest will get search results.