Suppose you are creating multiple websites based on particular countries and languages. Should you choose a .com and use subdomains to classify the variations? Use different sites completely? Use Top level Domains tied to the country you are targeting? Not much credible research has been posted on this topic, and as is often the case, it seems that Matt Cutts is speaking most clearly about this subject.
Highly respected (and deservedly-so) Search Engine Watch member Robert Charlton recently posted a couple of excellent comments in response to a thread about TLD's, their ability to affect ranking in foreign searches, and possible issues with duplicate content. Robert carefully explains the concept of mirror sites and localized markets, linking to some other topical threads. He then prescribes a good dose of 301 redirect. In a follow up post he points to the Mike Grehan interview of Matt Cutts, during which Matt suggested that keeping to the country TLD is very helpful for ranking within that country, although you can rank if your .com is hosted locally.
Robert also points to his practice of using country and language specific links in helping to rank sites in International versions of Google (and I assume other search engines when applicable). In any case, even if you don't comment, the thread is worth the read in order to get an understanding of this important topic, as well as for some good links to other related threads that tie it all together.
See the thread at Search Engine Watch Forums.