Ok, and now for my next question. How did you manage to obtain a 401K if you are a non-resident alien? If you were here long enough to obtain a 401K, it stands to reason that you were here long enough to meet the substantial presence test and be taxed as a citizen, although technically I suppose that doesn't matter.
Since the 401K is US Source income, you will need to file a 1040NR for each year you make withdrawals from the 401K, and may or may not owe taxes, depending on how much you withdraw.
The only income not taxable to non-resident aliens would be income that is NOT US Source.
Snipes