Here's an eye-opening article talking about a tool called Hamster that sniffs wireless traffic and reveals plain-text cookies which can then be used to impersonate users. The guy running the tool was able to log in into some poor soul's Gmail account during a BlackHat presentation.
Pretty scary, and it makes me think twice before firing up my laptop in a public wireless hotspot. The people who wrote Hamster, from Errata Security, already released another tool called Ferret, which intercepts juicy bits of information -- they call it 'information seepage'. You can see a presentation on Ferret here. They're supposed to release Hamster into the wild any day now.
Update: If the above wasn't enough to scare you, here's another set of wireless hacking tools called Karma (see the presentation appropriately called "All your layers are belong to us".)