And your XP desktop actually worked well with fast user switching? On an XP minspec machine? Or even recommended specs? There was a reason it was Powertoys only.

Not to mention that your XP machine had a HDD as a backing store for virtual memory. You could safely page out an entire session. Not so for phone OS's, AFAIK. (I know iOS doesn't have a backing store. Haven't checked Android, but I seriously doubt it)

Not to mention we've only very recently reached specs that make this doable. The iPhone 3GS (not that old) did have a 600MHz processor and 256MB of RAM. That's pretty close to an XP recommended spec, IIRC.

Given that your XP machine wasn't running on a battery, and wasn't busy in the background doing phone-y things, you can see where resources are getting tight.

Phones are just getting there, spec-wise. I'm sure sooner or later we will see a guest mode. The 'when', IMHO, hinges more on solving the UX issues, since specs always march on.