Hi,
first of all Im glad I found your site. You made your homework - thanks for sharing! There is a lot possible with iWeb it seems, but you have to dig deep and there are a lot of myths out there... so everyone is afraid of trying stuff out and taking the risk of loosing hard work/ already working websites.
I basically want to build on this topic: two computers, two mobilme accounts:
I have a running website (lots of content, lots of work) build with iWeb, published through my mobilme account.
A client saw it, he wants me to build a site for him, too. He has his own mobilme account and iWeb, so it should not be such a problem.
However I want to build his site on my computer, test it (through my mobil me account) and then transfer it to his computer and finally publish it on his mobilme account. So far so good.
Here is my plan and please tell me if Im running any risks here:
1. Back Up my iWeb = copy the files in admin->Library->Application Support->iWeb-> (Domain with 1.46GB, there is a second one called Domain1.sites with 12kb)
2. After the back up I take the Domain Files out of the iWeb folder. Now when I start iWeb, it should be empty for a fresh start.
3. Build client's website... NOW: what is happening in this mean-time? Is my website down for this time? Is it getting replaced by the new one I build?
4. Once the client's website is done I transfer the new Domain files to his computer and publish it - as you wrote, this should not be a problem (watch out for permissions, got it).
5. I copy my backup back into my iWeb folder (erase the client's one) ... do I have to republish my whole site now? Is there danger that my own site will not work and look as before?
(also - slight different question, but in context: it seems content that I take out when I edit my iWeb site is still stored on my mobilme, which I use to publish my site. Does that mean Im basically collecting all my trash there, but loose track of it, because more or less the stuff is a file package? When Im testing the client's site, does all his content stay on my mobilme account...?)
I have a ton of questions more, but lets start with this

thanks so much
daniel