You may wish to allow users to set their status to "Appear Offline". This was a status available by default in OCS 2007, but is not enabled by default in Lync Server 2010.
To enable the "Appear Offline" status for all users, run the following command from the Lync Server Management Shell:
Get-CSClientPolicy | Set-CSClientPolicy -EnableAppearOffline:$trueUsers will need to sign out and back into Lync to see the new status for the first time.
Strangely, users are unable to sign into Lync using the "Appear Offline" status. They must sign in first (preferably as "Away"), then set their status as "Appear Offline". Also be aware that this status is not available when users sign in using Exchange Outlook Web App (OWA).