All this FON sharing is well and good, but it's easy to forget that many (most?) UK ADSL broadband deals that offer 'unlimited' quantities of data throughput actually employ a 'fair use' policy of nGB. While most normal users will rarely, if ever, exceed a few GB/month themselves, they are far more likely to run into problems if they open their connection to others - even if the other users are not using P2P etc.
In addition to that, the idea of allowing other people to connect to the internet through your gateway is bonkers for a number of reasons. Most consumer devices won't hold extensive logs so you don't know what your 'users' are doing or where they have come from. The forensics team won't be able to tell either. So even if you don't get prosecuted, you could be partially (morally) responsible for enabling a crime. And even if a court won't find you guilty for someone else's abuse using your connection, imagine the stress and inconvenience of going through even the initial stages of the legal process.
Imagine also the stigma of being associated with certain online crimes (hacking, child porn, credit card fraud), even if you are not charged or if you are found not guilty. Will your girlfriend/wife/colleagues look at you in the same light again? "No smoke without fire," and other unfair thoughts...
Allowing random people to connect to your network also increases the chances of them knowingly or unknowingly introducing malware to your systems. Worms bouncing around behind your firewall is not a great situation, even if you use a good desktop AV product on your PCs (and Macs to a lesser degree). Yes, you could set up a DMZ - but honestly, why bother with the hassle just to give others free and unfettered access to the connection *you* pay for?
Compare this list of inconveniences to taking the short and basic security steps of enabling WPA, hiding the SSID (I know the latter is a fairly pointless defense against a knowledgable attacker) and not handing out passphrases to your neighbours. It's a no brainer and, in terms of risk/cost, very effective.
I appreciate that sharing can be a wholesome thing, but it does not always make sense.