I mean fast user switching on a resource-constrained portable device. There's a bit of a difference in terms of how to do it. (See also my above post listing some of the issues)
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.