This actually is a very serious problem-- so serious that the US Secret Service has set up a special project to fight it. The project is called "Nigerian Advance Fee Fraud - Operation 4-1-9" and you can visit http://www.secretservice.gov/financial_crimes.shtml#Nigerian
for a description of it. They have also set up a web page at http://www.secretservice.gov/alert419.shtml that gives details of the way the scam operates. Here are a few quotes from it:
"Unfortunately, there is a perception that no one is prone to enter into such an obviously suspicious relationship. However, a large number of victims are enticed into believing they have been singled out from the masses to share in multi-million dollar windfall profits for doing absolutely nothing. It is also a misconception that the victim's bank account is requested so the culprit can plunder it -- this is not the primary reason for the account request -- merely a signal they have hooked another victim."
"The goal of the criminal is to delude the target into thinking that he is being drawn into a very lucrative, albeit questionable, arrangement. The intended victim must be reassured and confident of the potential success of the deal. He will become the primary supporter of the scheme and willingly contribute a large amount of money when the deal is threatened. The term "when" is used because the con-within-the-con is the scheme will be threatened in order to persuade the victim to provide a large sum of money to save the venture."
"Victims are almost always requested to travel to Nigeria or a border country to complete a transaction. Individuals are often told that a visa will not be necessary to enter the country. The Nigerian con artists may then bribe airport officials to pass the victims through Immigration and Customs. Because it is a serious offense in Nigeria to enter without a valid visa, the victim's illegal entry may be used by the fraudsters as leverage to coerce the victims into releasing funds. Violence and threats of physical harm may be employed to further pressure victims. In June of 1995, an American was murdered in Lagos, Nigeria, while pursuing a 4-1-9 scam, and numerous other foreign nationals have been reported as missing."
"Indications are that Advance Fee Fraud grosses hundreds of millions of dollars annually and the losses are continuing to escalate. In all likelihood, there are victims who do not report their losses to authorities due to either fear or embarrassment."
It's better not to get involved with these people in any way. I always forward any of those emails I receive to 419.fcd@usss.treas.gov which is an address that has been set up to report this scam.