Just to add some explanations about the '2 months per year for 3 years' US residence raidenworks was talking about ealier in this post, it is really in fact approx. 4 months per year as year -1 only counts for 1/3 and year -2 only counts for 1/8 of days of presence.
If you calculate the 'average' maximum presence per year you can be in the US to *NOT* be considered tax resident for 3 years, it is 125 days per year (125+125/3+125/8=182,29), which is aprox. 4 months.
An average of 126 days per year would be too much ! (126+126/3+126/8=183,75)
Raidenworks (thanks!) kindly sent me the IRS link by private message about this:http://www.irs.gov/pub/irs-pdf/p519.pdf