Most wireless routers have built-in ability to encrypt signals. You may have heard of WEP, WPA, TKIP, etc. If there is no password on your router, it's extremely important to enable that.
Passwords are not all they're cracked up to be. All standard wireless encryption methods are extremely vulnerable to hackers. We had one of our network analysts try to hack every form of wireless encryption that wireless routers are capable of running with standard passwords set. The longest it took him to break in was an hour and the average time was about 1 minutes.
There are a lot of ways to improve this security so that even with the regular encryption it becomes next to impossible for people to break in. There are also additional layers of security that can be put in place.
Depending on how important your data is that is sent over the wireless and what the wireless network has access to on your office network, it may be a good idea to get the security improved.