Yes, it really doesn't make any sense to continue selling Vista much longer after Windows 7 comes out. I'd say that by the end of December system builders could make their final orders for Vista, with the last copies being shipped at around the end of January.
I guess that is what is going to happen anyway, even if there is no limitation on Microsoft's side. There will be no market demand for new copies of Vista being sold.
Windows 7 is built on the core of Vista, virtually everything that works on Vista runs as well if not better on 7, Windows 7 has the ability to virtualize XP from Professional Edition upwards at no extra cost to the end-user (compared to Vista anyway), and it runs better on low end machines than Vista. So where will the market demand be ? Businesses ? Perhaps, but most are still clinging on to XP for a good reason. Those that made the transition, will have all the time until de end of the year to test out 7 RC for incompatibilites over Vista, so very few will remain that will need a new copy of Vista by January next year. Of course, this however has nothing to do with the support lifecycle. On that topic Microsoft will of course provide all the patches and service packs until the end.
I bet there are a lot of people over at Microsoft that would love if 7 was made available to Vista users at a very SPECIAL price. I bet they are anxious to bring back their good image. I wouldn't say $7, but somewhere in the region of half the cost of Windows Vista Home Premium OEM. That would be a fair price, not too scandalous to anyone - neither Microsoft or the consumer.