Hey Rob,

I have a bit of exp with this - had a Canadian based site that wanted to target the states. We were ranking well for .CA and not so good in .COM. I actually did this in WMT for a site - set geo-targetting to USA - and after a week or so started noticing a huge jump in .COm for a lot of keywords. What was great was that the rankings in .CA stayed consistent.

The only drop I noticed was in the .CA (Canada Only) searches. These completely dropped off the map. But normal searches in google.ca were fine.Don't know if this will always happen, but this is my experience.