Arguably you could have the start/end dates at both levels! It really depends on the strategy you organisation takes. You might decide to have a particular promotion running for a finite period and then subsequently change the products that are included in that promotion. As you said Stephen, marketing departments come up with the most outlandish strategies some times.

I've been using temporal database design for a few years no and there's no question that it makes life easier down the track. I picked it up from a book 'The Data Model Resource Book' by Len Silverston which is well worth a read for any budding data modelers.