Hey Ian. As someone who dabbles in code rather than having time to get good at it, WordPress plus Thematic seems like an ideal platform for me. But I seem to have fallen at the first hurdle.
I uploaded Thematic, then created a separate new theme folder containing only a style.css file consisting of the header you provided (with the template link) followed by all the content of the original Thematic style.css (except the header), which I plan to tweak.
Both Thematic and my theme show up in WordPress admin. When I select Thematic, the preview looks exactly as I would expect; but when I select my theme, although it looks like Thematic, the sidebar is pushed to the bottom and various other styles are screwed up. Why would this be?