http://www.irs.gov/businesses/small/international/article/0,,id=129253,00.html

Conclusion

Nonresident alien students and scholars and alien employees of foreign governments and international organizations who, at the time of their arrival in the United States, intend to reside in the United States for longer than 1 year are subject to the 30% taxation on their capital gains during any tax year if during such tax year (usually calendar year) they are present in the United States for 183 days or more, unless a tax treaty provides for a lesser rate of taxation. This assumes that such capital gains are not effectively connected with the conduct of a United States trade or business. These capital gains would be reported on page 4 (not page 1) of Form 1040NR and would not be reported on a Schedule D because they are being taxed at a flat rate of 30% under I.R.C. § 871(a) or at a reduced flat rate under a tax treaty.

So 30% tax rate?