At our company we use open source software extensively on the development side including our IDEs (Eclipse), server OS (Linux), source control (SVN) and tech stacks (MySQL, Tomcat, Hibernate, Spring, Grails, GWT, RoR, etc.)
Our designers, on the other hand, don't use open source design tools such as GIMP
. This is the case for every design shop I know - interactive agencies, professional print designers, etc. Part of the reason for this is that the Adobe tools are highly refined. Photoshop, for example, has been incrementally improved by user interface experts for over 20 years. The devil is in the details, and when I've tried GIMP (admittedly its been about a year), it still feels half baked by comparison. The tablet support was spotty, I couldn't find common functionality easily, control over brushes was limited, no grouping of layers, no non-destructive filter support - the list goes on. Note that this isn't a knock on the developers. GIMP is much younger than Photoshop, and seems to be improving at a steady clip. Its just a long uphill battle against a company (Adobe) that, unlike some of the OS vendors, is actually doing a very good job of serving their market - designers.
Even if GIMP + Inkscape had a feature match for Adobe CS5 (the suite that includes Photoshop and Illustrator) and comparable UX design, it would still be an uphill battle for two reasons - 1) Adobe's tools cover every aspect of design and are very well integrated and 2) Designers are all trained on them in school. Programmers, by comparison, all graduate with experience using open source software tools.