A while ago (in 2005 actually), I blogged about how to disable HTML tab in the HTML Editor. How it is almost 2010 and it is time to post an update :-)
Well, first thing I would like to point out is that after 4+ years, the approach pretty much has not changed. We changed the rich text editor from CuteEditor to Telerik’s RAD Editor, released a great deal of new functionality, but still preserved the idea of backward compatibility even in such small things as rich text editor configuration!
Isn’t it amazing?
Enough reminiscing, let me cut to the chase! Today with 6.x, you can still disable HTML tab for the RTE by modifying the security permissions on the “HTML View” item in one of the rich text profiles, for example for the “Rich Text Full”: /sitecore/system/Settings/Html Editor Profiles/Rich Text Full/Buttons/HTML View
Two things changed here. All profile definitions migrated to the “core” database and now the container names are a bit different.
In addition to this, as one of our partners noticed, there is another button in Content Editor above the RTE field called “Edit HTML” which needs to have the same rules applied to. 
For that, follow the following steps:
1. Go to the core database, locate the field definition item:
/sitecore/system/Field types/Simple Types/Rich Text/Menu/Edit Html
2. On the Security Tab, click “Assign” and select the role you don’t want this option to be available to. Deny inheritance of this item or specify other security settings that make sense for your needs. 
That’s it, Folks! Enjoy Sitecore!