I am in precisely the same situation as you, and have researched that very topic extensively. The key question, as you have synthesized, is whether online poker qualifies as a US-source income.
Check out the charts on this page:
http://www.payroll.ucla.edu/alien/c1usin...es-nonresidents
Here's the problem: online gambling doesn't really fit into any of those categories (employment, or independent personal services are the closest, but even those aren't perfect matches). If indeed it does fit under one of the above two categories, then the source is determined by "where the services are performed". We hit another complication here: Is gambling really performing any "service"?? Not really -- if you were determined to qualify what the "service" actually is in this activity, then it's really the service the poker site and its servers are providing to the player! And those are located in a foreign country.
Regardless, I've talked informally to several lawyers and accountants about this, and most of them have told me that it's too grey an area to say either way, and there's definitely no case law on this yet. Also consider the tax laws of your home country. Even if you're not responsible for US taxes, you will be in your home country (if poker winnings are taxable there). Lastly, does your home country have a tax treaty with the US? Treaties are designed to prevent double-taxation...so if you DO declare your winnings on your home country tax forms, that MAY mitigate how harshly the IRS deals with you if they disagree. Just remember that most lawyers or accountants will tell you to err on the side of caution and pay taxes to the IRS on your winnings...but that doesn't necessarily mean it's the correct thing to do, rather that they think more often than not it's the easy / safe way out.