First, my network info: 2 desktops networked by Linksys Wireless-G router. Even tho' the router is wireless and wired, the two PC's are connected via cable. I plan to connect my laptop via wireless card when I can afford the card. I am aware that while I am not using the wireless capability now, the router is broadcasting wirelessly, none the less.
I have two questions:
1- Is there a way to detect if someone in my building is using my wireless connection to the internet?
2- Is there a problem if I just disconnect my telephone connection (DSL) while I am not on the web and just plug it back in when I want to use it? It would be easy to do with a simple coupler.

If you want security, encrypt using the router configuration screen and that is the end of your problem. Nobody will be able to access your signal unless they know your encryption base (like a password). Also set your SSID to not broadcast.

I have never used DSL (I only have cable), but what you describe will work with cable so I would guess that is would also work with DSL. Why not call your provider tech support and ask? You may have to reboot your DSL modem and/or router each time you do that, I am not sure.

You can go a step further while into the router access. In the Wireless section, you can select to Disable it until the time when you need it later on. You can select Linksys's security setup pages HERE.