there haven't been many recent WebmasterWorld discussions of this but it appears that setting the location preference in GWT is the strongest signal.
(for Google Search only obviously)
here are some recent threads you may want to look at:
Tell Google Your Site's Location with Webmaster Tools [webmasterworld.com]
Site Hosted Location and SEO [webmasterworld.com]
Google SEO and Location of the Webhost [webmasterworld.com]
Server IP vs domain extension in Geo-targeted SERPs [webmasterworld.com]

it's not clear at this point how much if any extra help you will get from IP (or IP block) location.