Arriving on St. Maarten / St Martin from North America currently requires a government issued ID and a birth certificate but on January 1, 2007 passports will be required from the US.  For US residents, Canadians and Europeans no Visa is required.  From South America it depends on the country.  All cruise ships dock on the Dutch (Antillean) side and passengers are pre-screened so have no immigration worries.  French St. Martin is a part of France similar to Hawaii for the US and all French laws apply.  The Dutch side is currently part of the Netherlands Antillies with Curacao, Bonaire and other islands but is scheduled to become independent in July of 2007 and laws may change.

Traveling between the Dutch and French sides is virtually unnoticable but remember to buckle you seat belt and put down the cell phone when on the French side as failure to do so can result in large (120 Euro) fines.

The island is completely duty free so don't worry about bringing in or out items (other than firearms or drugs of course).

 The only real concern is to have a return ticket.  Indefinite stays are not allowed and you will be refused entry.