I'm not a license expert, so I've got this little question.
I'm currently working on a theme for a WordPress-powered website for a company (which would make the website commercial, wouldn't it?). Now I'd like to use a SEO-theme (Thematic), which is licensed under GPL, as foundation for my own theme. Now my question is: am I allowed to use this theme as foundation for a commercial website? Or should I rather write a theme from scratch?