By hosting your own sites with multiple URLs your content will be seen to be more unique so links between your sites are given more weight. A site hosted in Australia might be given more weight for Australian content.
For regional (geo)targetting, you want to be hosted in one specific country. I would not want my sites hosted in multiple datacenters for precisely that reason-- it will screw up the localization of your site. It would be faster in speed and response time, but your IP should resolve to the country in which you want the traffic to come from... if you're selling to the UK, host in the UK. Selling to the US, host in the US, etc...
It definitely is with both Google and Yahoo. Yahoo even gives more weight if your inbound links come from a certain country-- if you are linked to from sites in the UK, you will do better on Yahoo! UK even if the target site is not in that country. I've seen this first hand from a few different tests.