Windows Small Business Server 2008 became available for purchase on November 12, 2008. Since then, I have spec'd several servers for customers and have yet to see the new OS as an option for purchase.
After a little digging, I found a post in their forums that says they are in fact selling the new OS - you simply have to request it over the phone.
As of this post, I still do not see it as an install option on their PowerEdge 2900 or 1900 series servers.
The Dell part numbers are A2108538 for standard and A2108545 for premium.
I am not sure WHY Dell has not updated their shopping cart to allow you to pre-configure this. I will post back when I learn the answer to that question.