Bruce Schneier has a wonderful essay up on Wired explaining why he runs an open wireless network at home -- and how that fits in with security. I've run open wireless networks since the late 1990s (in five cities in three countries) and I've never encountered the problems that everyone says are inevitable -- network contention, crap from my ISP, busts for the child-porn my neighbors are downloading from my network.
Instead, I've provided network access to innumerable people -- people like me: I can't count the number of times I've had my ass saved by an open wireless network at the right moment (e.g., in good time to help me look up directions, a phone number, or flight details). I figure the more open wireless I provide to the world, the more people I'll turn on to providing their own open wireless access, and the more open WiFi I'm likely to find.
To me, it's basic politeness. Providing internet access to guests is kind of like providing heat and electricity, or a hot cup of tea. But to some observers, it's both wrong and dangerous...
I remain unconvinced of this threat, though. The RIAA has conducted about 26,000 lawsuits, and there are more than 15 million music downloaders. Mark Mulligan of Jupiter Research said it best: "If you're a file sharer, you know that the likelihood of you being caught is very similar to that of being hit by an asteroid."
I'm also unmoved by those who say I'm putting my own data at risk, because hackers might park in front of my house, log on to my open network and eavesdrop on my internet traffic or break into my computers. This is true, but my computers are much more at risk when I use them on wireless networks in airports, coffee shops and other public places. If I configure my computer to be secure regardless of the network it's on, then it simply doesn't matter. And if my computer isn't secure on a public network, securing my own network isn't going to reduce my risk very much.