A significant part of sql server process memory has been paged out. This may result in a performance degradation.
I recently came across this message in the SQL Error Logs. This message indicates that another process, such as OS or another application on the database server is taking memory away from SQL Server.
Thankfully, finding the problem was a simple process for me (you may not be so lucky!). The error only occured out of hours and the times were relatively consistent. A quick call to the infrastructure team and I discovered they had turned on a replication process (HP StorageWorks Storage Mirroring) without letting me know. A quick rant and the process was disabled. I checked the logs the following morning and the errors had gone.
I had not seen the error before so I natually googled the issue and read a number of interesting forums and blogs. As my resolution was pretty straightforward, I have added some of the more interesting links that I read.
http://support.microsoft.com/kb/918483
http://blogs.msdn.com/psssql/archive/2007/05/31/the-sql-server-working-set-message.aspx