Ok since people aren't taking the hint in this thread, I'll go ahead and lock it.
I hope you'll indulge me a bit and spell this out, since I'm not sure that anyone made this crystal clear...
Apple's policy on Snow Leopard (10.6) is basically that it's an enhancement to 10.5. As such, it was sold as an upgrade at a steep discount of only $29 (here in the US). Normal major OS X releases historically ran around $129, so as you can see, this was a substantial discount. Also, 10.6 wasn't really a "feature" release, it was more about enhancing 64-bit support and tuning the OS to make it lighter and faster.
As a concession to those that didn't bother to upgrade to 10.5, Apple released "The Mac Box Set". The Box Set is actually a pretty good value in that it comes with both the latest iLife and iWork suites. Apple has expressly stated that the Box Set is their official upgrade path to 10.6 for users of 10.4. It was a little pricey upon release (I think the original cost was about $150), but has been substantially discounted by third parties like Amazon.com.
Now, with that said, it has been alluded to several times in this thread that the $29 Snow Leopard upgrade can be used to "upgrade" 10.4. While this is technically possible, Apple made it clear that it is unauthorized. While this forum does not enforce End User License Agreements, we do like to respect Apple's wishes - moreover, to-date OS X has been free of anti-piracy technologies like Product Activation (common to Windows) and other hassles that more often degrade the user experience for paying customers more than they stop piracy. It's our widely held belief amongst the staff that if the Mac community doesn't support Apple or tries to circumvent their wishes, eventually technologies like this may become integrated into OS X.
So, back to the original advice given by Schweb - the correct way to upgrade from 10.4 to 10.6 would be to purchase the Mac Box Set.
It remains to be seen whether Apple will offer a similar upgrade path for users stuck on 10.4-10.5 and not able to use the Mac App Store. I suppose if there is enough pent-up demand, they may eventually do just that. But if you're looking to upgrade to the latest and greatest right this minute, it's probably not a bad idea to pull the trigger on the Mac Box Set, even in advance of Lion.
My reasons for recommending this, aside from the legalities of doing so, are that if you're still stuck on 10.4, you've probably got an ancient version of iLife and possibly iWork too. These ancient versions likely won't run well on 10.7 (if at all). So, if you needed an added incentive to spend the extra $75-100 on the Box Set, there you go.
And Schweb, I know you intended to put the brakes on this discussion, so my apologies for continuing it - I just wanted to provide clarification for future users of the 'search' function.