Is that true for going between Photoshop and GIMP and Illustrator and Inkscape, or are they so different that it would be like going from MS Access to Oracle or from a tricycle to a Harley?!
I don't use Photoshop, but I still like to think I drive a harly. I graphics package is about 1/5 if that of designing a website. All you need is a basic graphics prorgam with guides, colour (gradient and solid colors) which easily allows you to insert photos and imagery as well as define dimentions. I know of people who use PhotoShop daily to design web pages, but don't have a clue about HTML or CSS. I hate to say it, but many use Dreamweaver to create tabled layouts on their sites.

Web design is focused on the technolgies we use. HTML, CSS, Optimization, JavaScript etc. Graphics software really don't count so much. I don't think it's good to add so much value on using a graphics package. This being said, I would also be very interested in learning something like Gimp or Inkscape, as I could truly focus on open-source.

PS: been using Open Office for 6+ years, would never consider using Ms Office again!

Edit: Macromedia lol! Last time I used their products was in 2004 if I remember correctly.