[quote=Ruby] would you ever consider having a 'guest GM'? [/quote]
I've found that I'm horrible at sharing GM-ship. However should I consider it, this would require a large amount of trust and familiarity. As for the first, there are only very few people I've seen capable of keeping an RP alive and a lot more claiming they can but ultimately failing to do so. Also the picking up the pieces-part isn't something that terribly excites me. As far as familiarity goes, you establish a style in a long-standing game; something players are used and drawn to. For example, I couldn't take over Kadaeux' Star Wars game, and I don't think he'd be able to substitute for the Library. Our styles are simply too different.

[quote]The second is more focused; would you ever consider inviting a RPer into your game (presuming the same as the above) to do 'spot work'; a villain here, an NPC there? If so, why? If not, why not?! I'd absolutely love to do 'spot work' for other peoples games. [/quote]
This would moreso be an option because it doesn't deal with power or as much responsibility. I've not done it, but it'd take some strain off the invitee in comparison to the guest-GM-ship. There would still be trust and familiarity requirements, but you'd work closer with these persons rather than just throwing them in the deep and expecting everything to go well.