A US citizen (and in most cases residents) owes US taxes regardless of where the taxable event occurs. It doesn't matter if you leave the US and NEVER come back. The only way to avoid US taxation is to renounce your citizenship (which generally requires you to already have citizenship in another country). Now if you are asking if you could "get away" with traveling to russia to do cash deals for rubbles and then laundering those funds though a bunch of offshore accounts? Maybe but that is asking "will I get caught", not "is this legal". Honestly if it is a small amount of money it probably isn't worth the trouble and if it is a huge amount of money speak to a good offshore lawyer and CPA. Tax avoidance not tax evasion.
Actually i was told that i can still keep my citizenship and only pay tax to the primary country if i only stay in US for less than xx days.
Many dual citizens do this.
I lived overseas for several years, and this is very clearly wrong.
If you are a US citizen (dual or otherwise) you owe taxes on all worldwide income, even if you do not enter the US for years.
There is a "foreign earned income exclusion" which you only qualify for if you are in the US for 30 days or less per calendar year. With the foreign exclusion you can receive a deduction of up to $97K (in 2013) on income earned outside of the US. This only applies to income earned outside the US.
Many US citizens who live outside the US earn less than the foreign exclusion and so believe they don't need to file US taxes or owe taxes, but this is incorrect. You still have to file a return each and every year to receive the foreign exclusion. If you do not the IRS can later stay you owe taxes and have lost the foreign exclusion deduction.
The dual citizens you know who earn less than $97K/year are correct that they don't owe US taxes, but they still have to file and pay taxes on amounts over that.