I actually reard in an HN comment that OS X's case-insensitivity by default was a choice that revolved solely around compatibility with Microsoft Office, so I think you're on to something.
Is that a combination of heard and read? I like it!
I believe case preservation but insensitivity has been in Mac OS since the original Macintosh File System; certainly it was in the Hierarchical File System and was inherited by HFS+. When they switched to OS X, they still had to maintain filesystem compatibility for the Classic system, and maintaining it for the entire OS probably made porting other Mac applications simpler as well.