Tim - Pan Macmillan, the publishers of Peter Hamilton, Neal Asher and others, sell ebooks directly from their UK website without DRM (
www.panmacmillan.com). They have DRM when you get them from other places, but not when you buy direct. They come in Epub format, I think, but without the DRM you should be able to format-shift using Calibre. They're a little pricier than if you buy them elsewhere, granted, but that's often the case with the physical books as well when you order them direct from the publisher.
BTW - I had that same message from directebooks at least one time myself, as I recall. I use a Mac Ibook running OSX 10.4, with Firefox.