It's a "way big" question(s) that you ask, one for which it's possible that today's strategy (advice) may cease to apply at any moment in the future.
The bottom line advice I have heard repeatedly is to avoid anything that is designed or calculated to game the ranking algo.
My mantra is this: If you are planning a business endeavor for the long term then making your website loveable by a search engine ought to be secondary to making a website that is loveable (worthy of link votes) by people. If your website is truly loved by people then chances are - or ought to be - that a search engine, one intent on having a future based upon its utility, will find reason (construct its algos) to love your website "as is".
When the engines can't get their algos to love (rank) websites that real people actually love then people will look elsewhere, besides that search engine, for guidance.
Go for people love. No kidding. Make your website a resource, link worthy without asking. Keep your SEO focused on usability, accessibility and keep your SEO efforts simple enough that even a dumb search engine algo will be able to figure things out. :)
[edited by: Webwork at 7:49 pm (utc) on April 27, 2006]
I'd argue that consolidation is the most secure option for the future because Google, in particular, sees the domain as the location of a site and is likely to do so indefinitely. Therefore the more lovable content you put on it, the better it will be regarded.
Steve
I sometimes get annoyed when the same company uses multiple domains. Forget about gaming the system - it makes me feel that they are gaming ME.
I'll give you an example. I was looking for a particular keyboard stand. I found it on Froogle at three stores:
homeofficewidgets.tld
officewidgets.tld
donthurtyourbackwidgets.tld
They were all the same company. Different shipping policies, different phone numbers, different mailing addresses, but when quizzed on the phone they admitted they were all the same company.
I bought from somebody else. It didn't give me a nice feeling about them.
They are targeting different market segments - I understand that. But, at the same time, they are gaming the system (and ME), getting 3 Froogle listings.
I see no problem with using subdomains, though. It makes it clear that the multiple sites are under common ownership. Yet it reinforces "this site is for some users, this other site is for other users". If you have widely-different product categories, this might be good.
home.officewidgets.tld
business.officewidgets.tld
donthurtmyback.officewidgets.tld
This is probably a poor example, and a marginal case for using subdomains. Your own situation is more compelling. But these guys are scumballs, and you are not. :)
If you have a recognized BRAND, though, that can be a good case for an entirely seperate domain name.
Think about your users - or customers - and what your domain naming scheme will make them think about you.