>This client has several disparate systems - or solutions.
>Each system use an app.config file. 90% of the data is the same for each solution.
>Is there any way to have one app.config file for all the solutions to use?

You could manually open a shared exe file with OpenExeConfiguration (http://msdn.microsoft.com/en-us/library/ms224437.aspx), or you could link the shared sections of your different config files to a shared file with the configSource attribute (http://msdn.microsoft.com/en-us/library/system.configuration.sectioninformation.configsource.aspx).