Ahh, bless you for posting that, Purple Wyrm, I was going to go looking for it if you hadn't. It's pretty helpful if you want to read in order, but as others have said, reading order isn't critical. I'm pretty sure my first Discworld book was Feet of Clay, and then I read a bunch of the other books that they have marked as "standard novels" (Jingo, Maskerade, Hogfather, Carpe Jugulum, to name a few) because that was what my library had. Over the years my brothers and I slowly amassed a collection of Discworld paperbacks to fill in the gaps.
That said, I think with some of the later books (especially the Tiffany books, which are marked on there as "young adult novels" in the Witches chain, and the last few books in the Industrial Revolution chain, starting with Going Postal) benefit from having read earlier books. For the Tiffany books, that's because they follow one main character through her growing up, and they often reference events in previous books. For the Industrial Revolution books, it's because they show advances in technology that have world-changing consequences, which I find interesting to see as it goes along. But, that said, it probably isn't crucial to read Making Money before you read Raising Steam, especially if you're already familiar with Going Postal. (Also, there's a movie of that? What?)
