Google enabled a way for webmasters to set a geographic target for their site. So if you want to tell Google, most of my visitors are from a specific area, you can do that. This way, if someone searches in a specific area and Google knows it, Google can serve up your site higher in the results for that user.
But what if a good portion of your target users are from a specific location but a large minority is not from within that area? Should you set your geographic area?
Personally, I would not. But a Google Groups thread has some more details from a Googler, Jonathan Simon. Jonathan said:
Setting your site's location information to Austria will help ensure that it will be returned in results for "pages from Österreich" searches. If your site is currently being returned for "the web" searches on google.de, it will continue to do so even after you've set your site's location information to Austria.
This does help but it doesn't give me the full picture.
There are more questions in the thread and hopefully we can learn more from Google's responses in that thread.
Forum discussion at Google Groups.

Comments:
Jehzeel Laurente
04/18/2009 11:27 pm
for me, i don't recommend geo targetting unless your only source of traffic is from that specific country. :) If you set geo targetting for US example, your serps from other countries may possibly get lower ranks.
No Name
12/30/2009 11:09 pm
I used to have in on Europe, but never got that much of traffic, now I have it back to the U.S., much better and my traffic has dramatically increase.
Mark Miner
12/31/2010 05:33 pm
Good to know