Themes affected:
Parent theme: Thematic 0.9.6.2
Child theme: Thematic Feature Site 1.0.1

Error Message:
Broken Themes
The following themes are installed but incomplete. Themes must have a stylesheet and a template.

Process that triggered the above error message:
Using Dreamweaver CS3 for Mac was the problem. Using DW to alter the child theme’s style.css file caused Thematic Feature Site to disappear from the WordPress Themes page.

Solution:
Used TextMate and Dashcode for Mac to alter the style.css file. Worked great.