This short article--followed by many comments--on techdirt brings up the reality that all the people who share their Internet connection via a wireless router might be setting themselves up for lawsuits by unwittingly aiding people in committing Internet crimes.
Unscrupulous people may leave their wireless LANs open to public access to cover up their own illegal tracks. The logic is that if their WLAN is open to the public, law enforcement can't prove with certainty that the illegal activity originated from the owner of the WLAN. However, finding the actual perpetrator of a crime will be all that's necessary to get a warrant for seizing all of the WLAN owner's computer equipment, after which the determination can usually be easily made.
There is also a mistaken impression that an open, wireless Internet connection somehow bestows immunity from liability via the Communications Decency Act (CDA) like it does for ISPs. However, sharing your Internet connection wirelessly with the neighborhood doesn't make you an ISP, and therefore doing so does not give protection under the CDA. And, as pointed out in the article's comments, the CDA offers no protection against intellectual property claims (e.g., copyright and trademark infringement) or criminal liability.
The real reason that you should not make your Internet connection wirelessly available to your entire apartment building or neighborhood is that your ISP's residential usage agreement probably doesn't allow you to this. It is likely that your ISP regards such activity as a sufficient reason to immediately and permanently terminate your Internet service.
http://www.techdirt.com/articles/20060320/1636238.shtml