The subsites module is probably what you are looking for. You can use the 1 CMS to manage multiple sites. For a real world example we have recently done is the epmu.org.nz site. This site uses the subsites module to control the http://www.workrights.org.nz/. You can also create, manage as many subsites as you want and set custom themes.
As to the state of it for 'general' release, It has no documentation and has had no public releases as of yet (only used on a few internally done sites) so its probably in the 'risky' to use category!
You might also be interested in the "Homepage for Domain" field, that each Page has on the "Behaviour"-tab within the CMS - which means you can have separate homepages for different domains. as long as those pages are not interlinked with each other (or searchable), they roughly act like separate websites.
Would it be possible to simulate this functionality by creating top-menu-level items each with a different .ss for its children's layouts, then redirect via .htaccess so that it jumps to the appropriate sub-page based on the domain? I'm only passingly familiar with apache's redirect capabilities, but I'm guessing the difficulty would be in supporting multiple pages within a domain.
I.e. if SS is set up with Domain1 and Domain2 top level pages with, say, Domain(x)Home and Domain(x)AboutUs children, would it be possible to set up redirects to map requests to the appropriate SS page, e.g: