The overall control of code hints for tag-based languages is in a file called C:\Program Files (x86)\Adobe\Adobe Dreamweaver CS6\configuration\TagLibraries\TagLibraries.vtm. It's in a similar location on other operating systems.
However, any changes to tag-based code hints are loaded from the TagLibraries folder in your personal configuration folder (see http://forums.adobe.com/thread/494812 for its location on different operating systems).
I suspect that what has happened is that whoever created the ColdFusion 10 code hints extension added the new CF tags to an old version of TagLibraries.vtm. That would explain why HTML5 code hints disappear. Dreamweaver always uses the version of TagLibraries.vtm in your personal configuration folder if it exists. As soon as you uninstall the CF10 code hints, Dreamweaver deletes TagLibraries.vtm from your personal configuration folder, and reverts to using the default version in the main program configuration folder.
Since you're comfortable coding with ColdFusion, you should probably have little difficulty editing the version of TagLibraries.vtm in your personal configuration folder. Just copy the first <taglibrary> element from the default TagLibraries.vtm and use it to replace the one created by the CF10 extension. You should then have both CF10 and HTML5 code hints.
I'm pretty sure that will fix the issue for you. But you should file a bug report concerning the extension. The ColdFusion team is completely separate from the DW one, and from what I hear, they don't communicate much. It should be a trivial matter for the CF team to update their extension.
Thanks David, I managed to fix the bug now. I combined the two TagLibraries.vtm files and made one proper one.
I submitted the bug twice to the Dreamweaver guys but if they are not in any communication with ColdFusion then I think that is enough confirmation to me that ColdFusion is on its death-bed.
Lack of communication between departments is common in large organizations for a variety of reasons. I wouldn't write off ColdFusion just yet.
I could understand if say the Illustrator team didn't communicate much with the ColdFusion team, but DW and CF should be like a married couple. You've got an amazing front-end web design tool which isn't in communication with an equally amazing back-end language and they're both made by the same company? At the same time they offshored all of CF to India therefore effectively isolating it from the main HQ. To me it sounds as if CF might eventually be sold off to another company.
Married couples don't always get on. What I heard from an Adobe insider is that the ColdFusion team would prefer everyone to switch to ColdFusion Builder. As for the development of CF moving to India, that's true of many Adobe products, including Dreamweaver.