Themes affected:
Parent theme: Thematic
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.

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