Hardware is indeed pretty much alike, but nowhere the same-same as the Windows operating system across the Govt.
Why would a bad guy go for the tougher-to-reach hardware when a complex, low-security OS is sitting right there running wel-known applicaitions (IE browser, Adobe, etc.) ready to exploit? Bruce Schneier writes of Software Monoculture (
http://www.schneier.com/blog/archives/2010/12/software_monocu.html). In the near term, a better thing to do would be deploy a few flavors of locked-down OS's (say Windows + Red Hat Linux + Mac) so only a third or so of the network could crash at once.
DARPA seeks the far future (Go DARP!)... but we in the closer-to-now, practical world should pick these lower fruits.
Secondly, adding hardware diversity could be as simple using the widely deployed Trusted Platform Module (TPM) and add-on 'unique' encrypted hardware that are locked to specific applications.
