Sure, there are a few people that say things like "Why should I have to pay Apple to fix the mistakes they made in Leopard?" On some level, that might be a valid argument, but let's set that aside because there's a better point to be made.
The handful of new features in Snow Leopard are well worth the price Apple is asking. Included are most of the QuickTime Pro features, text substitution, more expansive Spotlight integration, Expose in the Dock (sounds like a band name... hehe), and a host of other little improvements. Some of these can be added today via third-party apps, but those apps (*gasp*) cost money. Bundle all of those up, and $30 for the whole slew of apps would be a sweet deal in and of itself. I know I would spend half that just to get the new slider in Finder windows that changes the size of thumbnail previews!
Heck, the reported speed improvements make the shipped price of $29 per machine price (which, remember, drops to below $10 with a Family Pack) a bargain. Even if the overall increase turns out to be low, say 5% or so, the price of the OS upgrade turns out to be a good deal. Where else are you going to spend so little money for such a similar boost in speed and snappiness?
And remember, that $49 lets me upgrade five Macs, not just the one PC I can update to Windows 7 for over two and half times the price! (Microsoft is asking $129 for the Windows Home Premium Upgrade.) A friend of mine was excited to find out that he only needed to spend $150 to upgrade his three Vista machines to Windows 7, using Microsoft's family pack. I understand his enthusiasm: I'd gladly pay $150 to get rid of Vista, too!
Seriously, Apple did a very smart thing when it priced Snow Leopard below the cost of a typical family visit to the movies. If it would have asked the normal going rate of $129, I would probably have still upgraded, but I, along with millions of other users, would not have been happy about it.
Keeping the iPhone 3G around at $99, lowering the prices on new Macs, and now the awesome deal on Snow Leopard... I think Apple may, just may, be "getting it" when it comes to pricing lately.