Yes, you can go from 10.4 to 10.6 without installing 10.5. The $29.00 disc works just fine, I did it on my "new" ('07 Intel) mini. As far as the issue about whether you can "officially" use the $29 disc to go directly from Tiger to Snow Leopard. I looked very carefully at the license agreement and noted that it stated that if you had purchased it as an upgrade you needed a license of Leopard installed first. I also looked very carefully at the box, the barcode label, and the disc. The box and label declare that you are buying "Mac OSX Snow Leopard", and the disc says it is an "Install DVD". Nowhere could I find anything that implied that the disc was only for upgrading.
The official "Box Set" upgrade deal does include a nice upgrade to iPhoto and some other things and looks like a good deal for the price (compare to upgrading Windows). I would have gone that route myself, if I had not just gone through the effort and cost of tracking down a legitimate copy of 10.5 to update my "old" ('05 G4) mini from Panther (that was enough work that I didn't feel too guilty just buying the $29 10.6 disc).
The impression I get is that Apple would like to sell you upgrades to your iPhoto, iLife and the rest of it too. But, they would also like to get as many people using Snow Leopard as they can. Consequently, they don't make the Box Set the only way to move from Tiger to Leopard (I'm sure the $29 disc could have easily been configured to look for a Leopard license). They would also like people to move away from the older G4 Macs, hence the difficulty in getting Leopard.
If anyone out there wants 10.5, I got it from Apple for about $140 (last November), but not at the Apple store or on their website, I called one of their 800#s (I'll look it up if anyone really wants to know). Amazon, eBay etc. have such a mish-mash of prices and versions that, combined with uncertainty in knowing what might actually be delivered, I found buying from Apple was the easiest way to go.
