Recent decision on Fed's computer search activities may have impact for future library records seizures -
"Feds seizure of baseball players' drug tests ruled illegal" http://www.cnn.com/2009/US/08/26/steroids.ruling/index.html

Seems that the Feds had warrants and subpoenas for 10 players' database information, but they took 104 players-worth of drug test results.

To this non-lawyer librarian, it would follow that if the Feds were asking for records on a specific library patron, they had better not mess with the other patrons' information.