By default, Agent SVN will mark the file as writable whenever the file is checked out and mark the file as read only whenever the file is checked in.
This feature is designed to emulate the file management behaviour of other source control system, for example Visual Source Safe.
This feature can be disabled by unselecting the mark file as read only option found in the Agent SVN configuration dialog.
It should be noted that some IDEs' do their own file management, for example Visual Studio and in these cases this option will have no effect.