Parker on meth
Wil McCarthy has a cute article on "The Physics of Spider-Man" at SF Weekly. He discusses super-strength, wall-crawling, web-swinging, and spider-sense, and hints at the possibility that Spider-Man may be a drug addict.
At the bottom of his arc on a 30-meter webline, Spidey's hand will be supporting up to three times his own weight. Without truly superior strength and clinging power, our hero is set for an ignominious tumble to the streets below. Spider silk is also highly elastic, and assuming a thickness of half a centimeter and a Spider-Man mass of 70 kg, the 30m webline (if it's actual silk) will stretch to double its length at the bottom of the swing.
McCarthy also mentions the real-life climbing device known as the Gekkomat, which uses computer-controlled air pumps to create a vacuum on much rougher surfaces that would be possible with plain suction cups.