Expanding on the above:
Even if you bought a MacBook Pro today, you could not reinstall 10.6 on it using your older install disc. You'd need to use the disc included in it of 10.6.x (I can't remember the number -- whatever was current at the time they came out) that includes the newer drivers the machine needs to run. Any Snow Leopard install discs that contain versions newer than 10.6.x should work, though.
If you bought a MBP a week after Lion's release, and it's running Lion out of the box, it's recovery discs (or recovery partition, if they switch to that) will also contain Lion. If you desperately wanted to revert to 10.6, and assuming there's no changes to the MBP hardware between now and then (which there won't be, trust me), then your best bet would be to get ahold of the recovery discs that would have been included with it before Lion's release. (e.g., borrow them from a friend who bought a MBP between February and now).
Or, just buy the MBP now; it will be running SL, and contain those discs, but it will also come with a free upgrade to Lion when it's released. That's the most hassle-free way to hedge your bets.
Any new hardware released on/after Lion's release will never run Snow Leopard, because there will never be a version of Snow Leopard that includes all of the drivers they need to run. That could be the real reason Apple might be holding off on the Air and mini releases (assuming the rumour of this article is true): it saves them the trouble of having to add and test any new GPU, motherboard, or Thunderbolt drivers these machines might require to both Snow Leopard and Lion.