I think a lot depends on where you live. Urban areas are perhaps less likely to arrest an individual for simple possession while the likelihood goes up in rural areas.
Especially here in Canada, where enforcement has become spotty - the application of the law is fairly arbitrary. None of my friends here in Toronto have faced charges for marijuana, even under contexts where you think it'd be a sure thing. Its a different story in rural Alberta of course...
While I'm sure America also has an unequal application of the law it appears
that marijuana arrests are actually on the increase:
[qimg]http://norml.org/share/marijuana_arrests_chart.gif[/qimg]
And
according to this it appears as though 88% of the 830 000 arrested under marijuana laws in 2006 were for personal possession and use, not trafficking. 47% of all drug arrests are for marijuana.
It is likely you live in one of America's "islands of lax enforcement".