The users I talk to want their data centers simplified -- but they still want best of breed. They are once again being asked to do more with less and they are looking to the vendors to help. But I don't think most of them want that to mean that all of the technology has to come from one vendor in a single box that does everything. Most users still want the ability to pick the best of breed for their unique situation, they just want the products to work together better.
There are times where getting several of the components from a single vendor make sense. Brocade, for example, has a case to make with the common HBA and switch strategy. CommVault has a case to make with backup and archive being integrated.
There are times when complimentary components may be more compelling. Symantec, for example, has an equally interesting play with their Open Storage Option (OST) and dedupe vendors like Sepaton and Data Domain are taking advantage of the option to provide better performance and integration into NetBackup.
I can't think of a single instance where getting the entire system stack from a vendor is going to leave you with an unquestioned best of breed solution or even a stack made up of second bests. Most vendors don't offer the complete stack yet and those that do begin to show weakness in their stack the further they get away from their core competency. Most vendors can do one product category really well, two categories pretty well and then fall apart after that.