Knowledge Base documents:

When launching Revit you are unable to obtain a network license on a specific workstation but other workstations can obtain a license.
Revit network license troubleshooting

After launching Revit 2010 products a license error occurs and the Product and License Information displays an error 17 status.
Revit 2010 error 17 license status troubleshooting

When you launch an AutoCAD 2010-based product it is able to acquire a license but a Revit 2010 product is unable to acquire a network license.
AutoCAD 2010 products are able to acquire a network license but Revit 2010 products cannot

After you reinstall an Autodesk 2010 product redundant licensing no longer functions for Revit.
After reinstalling an Autodesk 2010 product Revit redundant licensing no longer functions

After launching Revit 2009 64-bit products you receive an Unable to obtain a floating license error at the Recent Files page.
Revit 2009 64-bit products display floating license error after startup

Revit 2010 is not able to recover a lost network license when Revit is running. The program switches to Demo mode and any open data files will not be saved.
Revit 2010 License Timeout Workaround - Update 2 and Subscription Advantage Pack Build

Create an environmental variable for FLEXLM_TIMEOUT
Error: A valid license could not be obtained by the network license manager

This can also be useful: Clean uninstall of Revit 2010 products describing what folder, files and registry locations that needs to be removed.

For reports on license utilization and usage see JTB FlexReport.