another point most people dont know about, is that if someone (unauthorized) is using your wireless connection to commit any sort of offences, like hacking, illegal file sharing or even browsing for prohibited content , this will be traced back to your address, thus possibily putting you in a serious situation, unless you can prove it wasnt you ( quite hard if you dont have the knowledge or the detailed logs)
the advice is : encrypt your connection using WPA-128bits minimum ( DONT use WEP) it take minutes to crack , choose a strong key, with numbers and uppercase & lowecase caracters and if possible use MAC address locking ( see your router manual, its all there)
If you leave for holidays, turn the router off !
Its a bit like car crime, simple steps go a long way ...