How secure is Mac OS X's security? My friend's MacBook Pro was recently stolen. Fortunately, it was protected with a password. How secure is the built-in protection ?

Can thieves access the data on the hard drive, and at what difficulty?
It depends on exactly what you mean by: 


  it was protected with a password


If it means that they were using FileVault, then their data should be pretty much inaccessible.

If it just meant that they had a login password, though, all the thieves have to do is boot the laptop off another disk, and they'll have immediate access to everything on the internal drive.