That depends on what you're doing with the domain names. If you're having multiple domains linking to each other on the same server (and IP address), then you'll be operating a link farm as far as the search engines are concerned, and when they find out, all of your sites will probably be banned. (I had recently completed an SEO analysis for a client where I had found nearly two dozen of his top competitors attempting to manipulate the search engines - two of them were operating link farms). If the domains on the other hand are just minding their own business and gathering backlinks naturally (in addition to appropriate and relevant submissions), then they'll be fine and will be treated like any other domain.
Nope. Quite simply, Google treats subdomains and directories (both of which contain pages) to be on the same domain these days. If people had taken my advice and used subdomains appropriately instead of using them to manipulate the search engines, Google probably would not have had to take such drastic action to begin with. * Note: For the record, Google has always considered directories to be on the same domain; until last year however, they treated subdomains to be as "separate domains for SEO purposes" - I always told people to use these to network a collection of "mini-sites" that were related to the same site to each other, such as how Yahoo handles its portal with sports.yahoo.com finance.yahoo.com weather.yahoo.com and so forth.
Not always, n3o. If you're going to be creating multiple Web sites that are inter-linking heavily to each other, then yes, you might want to have different servers with different Class C IP addresses, but it'll still be easy for Google to tell that you own them all - afterall, they ARE a domain registar and they DO have access to the Whois database.