How strong is spider-man's webbing?

According to the films his webbing is as strong as high-tensile steel. Which has a yield strength range of 50,000 to 100,000 pounds per square inch. We see how strong it is in the films when it holds up an entire trame full of people in the first film, and stops a run away train in the second. It shows that it is even strong enough to pull a brick wall apart in the second film when he doesn't apply enough to stop the train.

http://www.answers.com/topic/high-tensile-steel

While in the comics Peter Parker made his own webbing and thus is as strong as Peter (or the writers) wants it to be. The webbing has gotten stronger over the years. More than likely due to the fact he has perfected the chemical make-up of the webbing. It has been shown to be weak enough in early years to be snapped apart by Doctor Octopus with little effort. While recently it has been shown to be strong enough to hold entire buildings together, or hold characters as strong as the HULK and Iron Man captive (for a short time at least.)