The SQL Server 2008 R2 Parallel Data Warehouse Edition is a new edition of SQL Server that will be released in the autumn. Essentially its a series of SQL Server machines controlled by a head node that decomposes queries and runs them across the machines then consolidates the reults and returns them to the user. Its designed to cope with workloads in the 10's to 100's of Tb region.
By scaling out rather than up we can create a powerful system at a far lower cost. So in my view its at least worth considering before you make a decision (assuming you need this scale of solution). You should also look at the Fasttrack solutions as well.
Have a look at:-