Hi WillTravel --
While there have been comments on this message boards, and I am sure others, about the pickpockets in Madrid... on the metro and in the city center, I can assure you that it is not exclusive to Madrid, nor is it so wild and rampant as some of the posts will have you believe.
I have lived here for the past four years and have yet to experience an incident of pickpocketing or robbery (knock on wood)... and I use public transport all the time, including the infamous bus 27 which is notorious for its crafty passengers.
Regarding the metro to and from the airport. If you are thinking about taking it from the airport when you first arrive in Madrid (from Vancouver) you might want to consider another option....if anything...for the jet lag. It´s a distance from the international terminal, and depending on your ultimate destination, you may have to change lines once or twice. But that's just my opinion!
The airport line in Madrid is new and modern with closed circuit TV and airconditioning. However you might have to connect with an older line (such as line 1) which does not have airconditioning. Given that you plan to be here in August, I think ridership on the metro will be much lighter since half of Madrid will be at the beach and the other half will be in the mountains on vacation!