According to Process Explorer running on XP x64, native 64-bit processes get "Hardware" DEP status, with the default Windows settings 32-bit processes get "None".
If you launch a 32-bit app which spawns a 64-bit process, then the 32-bit app gets "None" for DEP, and the spawned 64-bit process gets "Software" for DEP.
(An example is cmd.exe launched by the Sourceforge app "Console" as an alternative command prompt.)
"Hardware" DEP is not limited to Windows processes alone, Ultramon 2.6, Process Explorer, File Monitor, Registry Monitor and the ATI Catalyst control panel processes all get "Hardware" for DEP status.
Change the default Windows DEP settings from:
"Turn on DEP for essential Windows programs and services only"
"Turn on DEP for all programs and services except those I select"
and all 32-bit processes get "Software" for their DEP status.
Some of the apps/processes I tested were...
Avast! 4.6 (ashDisp, ashMaiSv, ashServ, ashWebSv, aswUpdSv)
Tiny Firewall 64 (amon, rtshoo~1, UmxAgent, AmxCfg, AmxFwHlp, umxlu, UmxPol, UmxTray)
Internet Explorer 32-bit
ATI Catalyst control panel (ati2evxx, atiptaxx)
Ultramon 2.6 (UltraMon, UltraMonTaskbar)
Internet Explorer 64-bit
cmd.exe (plus all Windows processes)
So there does appear to me to be a functional difference in the degree of DEP support in 32-bit and 64-bit processes.
I believe the emulation of 32-bit apps on a 64-bit OS is at the OS level, not the hardware level - the CPU architecture has no bearing on it (it just allows 32-bit OS's to run natively as well as 64-bit OS's).