isight -- when we say "retail disk" we mean the one that came in a box and has a picture of a snow leapard printed on it. The "restore disks" are the grey disks that came with a particular machine. For your purposes, the "restore disks" are the grey disks that came with your 2009 iMac.
The grey disks which came with your 2009 iMac certainly don't have thuderbolt drivers, not sure about displayport drivers.
The retail disks, which sometimes are 10.6.0 and other times 10.6.3 do not have thuderbolt drivers, but certainly DO have displayport drivers -- the retail disk contains every driver for every possible piece of hardware that runs that particular operating system.
The thunderbolt drivers are in the 10.6.7 update (or the 10.6.8 combo update.) 10.6.7 was what the first thunderbolt machines were shipped with.
What we do not know is whether those 10.6.7/10.6.8 thunderbolt drivers will drive the thunderbolt on the mac mini, or if they only work on a 2011 thunderbolt iMac. When Apple says that a machine will not work with an operating system lower than waht it shipped with, they mean that they didn't test the new machine with the old OS (old OSs plural) and so who knows what will happen? If those minis had come out before Lion was ready, then the development team would have had to have gotten the thunderbolt drivers to work correctly on them. There may have even had to have been a 10.6.9 to make them work.
What seems most likely is that most/all "old" features of the 2011 mini will work just fine with SL. The things most likely not to work would be the "new" stuff -- in other words, thunderbolt. We already know that it doesn't work for at least one person. But it may only be broken for certain thunderbolt devices, too, and others should work ok.
The painful conclusion is that if you MUST have an i5 or i7 processor and/or thunderbolt, and you MUST be able to run on 10.6, then your only guaranteed hardware choice is a 2011 iMac. For those of us dealing with size/space/heat constraints (i.e. we need to use this back in the server closet!) the iMac form factor is a problem. This exercise is useful, because we are, through experimenting with different hardwars, figuring out just which things will still work right on an i5/i7 mac mini running snow leopard, and which things are broken.