ecntrc, brushes placed in the Adobe Photoshop Only folder can only be used in Photoshop. However, if the brush filelibrary is an ".abr" extension file and NOT placed in the
PS Only folder, it can be used in ImageReady also. As far as Illustrator goes, I have only been able to use file extension ".ai" libraries.
Just to confuse you a little more....if you use Paint Shop Pro, some of the brush libraries can be used in
PS by changing the file extension. So unless the brush file/library states it is for use by
PS Only, I always place it outside that folder.
Another little quirk to consider are file versions. Often a brush file/library will work only in a specific version of PS. Just another little thing to keep in mind.
You say you are using CS version, was it worth the upgrade? I am still working in
PS 7 and just don't have the cash yet for upgrade. Am thinking will wait little longer.