I’m a network engineer with 30 years of experience. Penetration testing and computer forensics of large networks is what I do for a living.

First, let me scare you. Nothing is secure from an expert, even when all indications say it is secure. When you know it’s unsecured, you are at greator risk from the script kiddies, and they could potentially do anything on your network. Go out via your IP, make configuration changes on your workstations, change the code on your router and or cable/DSL modem, plant malware on your switch, or printer. Yes, I said printer.

Now lets talk probability. Odds are, the range of your network is limited so that only your imediate neighbors could utilize it. What are the odds that your imediate neighbors are experts or even script kiddies? Having your network open can actually protect you legally. If it’s not secure, and illegal activity is taking place then you COULD have plausable deniability

If somebody like me were war driving, we look for a place we can park and can blend right in, and maybe access multiple networks, like in a area of several hotels.

You likely do not have too much to worry about.

Well the only serious security threat I can think of is of infringement. The perpetrator can enter your unsecure wireless network and perform illegal activities, under your IP address. Thus, you’ll be liable for in lawsuits even though you’re not at fault. Common illegal practices include music download, movie download, file-sharing, etc.

WPA/WEP encryption makes it hard for average users to get into your network. The MAC address adds another security layer, but even that’s not enough to stop an experienced hacker. However, home networks aren’t usually the targets of hackers (usually businesses or government agencies)

Well theres nothing really to worry about other than him looking at porn, but roommate? your old enough it sounds like to be doing so, i dont see a problem in it he cant mess with your connection or nothing though.