I was hoping that the Adobe development team would have fixed this by now but it seems that they do not use their own products. At least, certainly not for standards compliant work. In a revisit to my original article on fixing the issue with CS4, here is an updated tag library that you can use to stop Dreamweaver CS5 from changing your correct onmouseover events to the improper onMouseOver mixed cased version.
Download the HTML Tag Library and overwrite the existing one which you’ll find by default in “C:\Program Files\Adobe\Adobe Dreamweaver CS5\configuration\TagLibraries” on x86 systems and in “C:\Program Files (x86)\Adobe\Adobe Dreamweaver CS5\configuration\TagLibraries” on x64 systems.
It annoys me that this hasn’t been fixed yet. Thanks a lot for sharing your quick fix.