Do I *have* to purchase the Snow Leopard box set?

"The snow leopard install DVD is identical, there are no differences technically, though the standard suggestion is that the EULA changes:
Apple's Snow Leopard disc will install on Tiger Macs
Actually, there is currently no clear information regarding whether upgrading from Tiger is really ""illegal"", both Walt Mossberg of WSJ and David Pogue of the NYT have both suggested to do so. I've not read the actual EULA (couldn't find it on the Apple site, have yet to go get my SL disk from the Apple store yet) so can't state what the precise position is, people who have said this is ""illegal"" have not justified it with links to the EULA itself, just assumed it is in there...
And as I understand it, breaking a EULA is not illegal, but is a breach of contract.
OK, here is the bit of the EULA from my Snow Leopard, and terms and conditions clearly state a previous valid Leopard install. What is interesting is that nowhere on my box or disk or instructions can I find it saying it is a ""Leopard upgrade"". Even my till reciept call it ""Mac OS X Snow Leopard 10.6"", so apart from the price, even my packaging is a single install.

Permitted License Uses and Restrictions.

A. Single Use License. Subject to the terms and conditions of this License, unless you have purchased a Family Pack or Upgrade license for the Apple Software, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer at a time. You agree not to install, use or run the Apple Software on any non-Apple-branded computer, or to enable others to do so. This License does not allow the Apple Software to exist on more than one computer at a time, and you may not make the Apple Software available over a network where it could be used by multiple computers at the same time.
B. Family Pack License. If you have purchased a Family Pack license, then subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on up to a maximum of five (5) Apple-branded computers at a time as long as those computers are located in the same household and used by persons who occupy that same household. By ""household"" we mean a person or persons who share the same housing unit such as a home, apartment, mobile home or condominium, but shall also extend to student members who are primary residents of that household but residing at a separate on-campus location. The Family Pack License does not extend to business or commercial users.
C. Leopard Upgrade Licenses. If you have purchased an Upgrade for Mac OS X Leopard license, then subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on a single Apple-branded computer as long as that computer has a properly licensed copy of Mac OS X Leopard already installed on it. If you have purchased a Family Pack Upgrade for Mac OS X Leopard license, then subject to the terms and conditions of this License, you are granted a limited non-exclusive license to install, use and run one (1) copy of the Apple Software on up to a maximum of five (5) Apple-branded computers at a time as long as those computers are located in the same household (as defined above), are used by persons who occupy that same household, and each such computer has a properly licensed copy of Mac OS X Leopard already installed on it. The Family Pack Upgrade for Mac OS X Leopard License does not extend to business or commercial users.

So Apple really is leaving it to the conscience of the user to ""know"" his copy is an upgrade..."

"I just noticed some very interesting details in the Snow Leopard Wikipedia article, and wanted to add them here.

Turns out that not only does it technically work, but you are not even breaking the EULA if you upgrade from 10.4 Tiger with the $29 ""upgrade"" version.

From License section of that article:

Single Use: places no restriction on
which (if any) version of Mac OS X
should already be installed

[...]

The products advertised as 'Mac OS X
10.6 Snow Leopard' ($29) and the 'Mac Box Set' ($169) both come with the
same Single Use license. This is
particularly relevant for Mac OS X
10.4 users, for whom it is legal (under the terms of the EULA) to
install either the $29 or the $169
product.

Meanwhile, the ""Leopard Upgrade"" license (which indeed states that 10.5 is required) is only used for the Up-To-Date program, which is relevant just for those (like me) who've bought a new Mac with Leopard recently and want to upgrade to Snow Leopard for $9.95.

Edit: On closer reading, I think the above Wikipedia information is pretty much verified by what The Tentacle wrote in his answer:

What is interesting is that nowhere on my box or disk or instructions can I find it saying it is a ""Leopard upgrade"". Even my till reciept call it ""Mac OS X Snow Leopard 10.6"", so apart from the price, even my packaging is a single install.

Just that the bolded parts of the EULA (emphasis added by joshhunt) are not relevant to him. :-)

I should receive my Up-To-Date program copy of SL probably this week, so I'll be able to check if that indeed uses the ""Leopard Upgrade"" license."