There's quite a bit of widely used professional software that can't use more than 4 cores. Some such as PhotoShop, can currently only utlize 2. This will change over time, but it will be awhile IMO, which would translate into at least one generation of computer later.
Same thing with memory bandwidth.
So decisions will have to really focus on specfic use, as those that have purchased or considered purchasing the '09's discovered. Cost considerations have become more prominent whether it's an individual pro or corporate entity, and will be an even greater factor in which machine is the most viable solution IMO, as budgets have been tightened (i.e. including how/what to upgrade, such as going for additional RAM and/or RAID vs. higher clock speeds for the CPU's to obtain the greatest overall performance value).