This is a funny topic, and I've wondered what other people thought about this. I've witnessed both: people using their bags to hold their place in line, and people coming from the side trying to "blend in" to the front of the line. I usually am not bothered by these, but if I'm in a particularly feisty mood, I just act cheerfully unaware. I've "ignored" the people trying to blend in and walked past them, and I've also gotten in line in FRONT of the bags, figuring if anyone said anything I'd just say, "I didn't realize you were in line -- I thought these were just sitting there." (Nobody said anything. In fact, other people got behind me, in front of the bag.)

I guess if you really wanted to be difficult, you could call airport security and mention the unattended bag. Although I really did see an unattended bag in SAN once, and when I contacted security they didn't seem like they knew what to do...

And pnut, you're right -- many policies are inconsistently enforced.