I have heard in a few place that is is possible for dishonest people to change your affiliate id to theirs and re-direct people who sign up through banners so they get credit. I check mine daily and have not had a problem but wanted to know from people here if this is something that can happen since you guys are the most knowledgeable.

Well yes. For example I am a member of amazon's affiliate program. Say I was searching for something and came through your site and saw you advertising a product I liked. I could always click the link but then replace your affiliate id with mine so I would get the commission instead of you.

I think this is what you are talking about. Of course people could also do this if they run a banner network or something, they could write a script that would change the aff. ids to theirs, although most networks wouldn't do that as it would sooner or later kill their network.