I’m the same as most of the previous posters- as long as you say something, I’ll even watch your cart and make sure nobody adds or subtracts. 
If there’s an unattended cart at the end of a line, I’ll pick another register, if that’s not possible, I’ll get in front of the cart but not push it out of the way. If the shopper comes back within a minute or two, I’ll offer to let them back in. Nine times out of ten, they’re just glad the cart is still there.
If it the unattended cart comes to the front of the line in any of these circumstances, I’ll push it aside and put my stuff on the conveyor. I generally don’t leave the line, and if I do it’s only when there is a long and slow-moving line ahead of me (like at least 4 carts between me and the one being checked out. Also, I make sure to ask the person in front of me or behind me to watch the cart so it doesn’t get caught in a re-shop sweep.
long, pointless story ahead****
The last time that I was in this situation, it turned an otherwise tedious trip kind of fun. I was at the semi-trashy discount supermarket near my house* on a sunday afternoon, which is the worst time to go to any grocery store (after-church crowd) but is always brutal at this place- and it wasn’t until I got in line that I saw that there was only one cashier on duty. There were 50 people in line. Even when they got a second guy on (I think they had to call him in) the line was still not moving. If I had had anywhere to be I would have just left, but I wanted my chicken for dinner more than I wanted to go home, so I decided to stick it out. A young woman got in line behind me who only had sour cream, and we joked about that for a bit. Then the guy in front of me asked me to watch his cart while he went to grab something, which I did. Then the woman’s husband showed up to find out what had happened to her (we had been in line for almost a half an hour at this point and she had left her cell phone at home).
Something in their conversation reminded me that I had forgotten carrots for the soup that I’d be making with the chicken carcass. I asked them to watch my cart, offering to switch places with them when I got back (which I had planned to do anyhow, since they were just getting the one thing). They waved this off, and I went and grabbed the carrots. There was also some nice looking spinach, so I grabbed that, too. When I got back, they were amazed at how nice the veggies looked. Apparently it had never occurred to them that their local trash-mart had a decent produce section. My parents were amazed too. It just goes to show that you shouldn’t judge a grocery store by your prejudices about its customers. To be fair, other stores in this chain that I’ve visited have been pretty crappy, with no produce at all, but still- don’t you even look?
Anyhow, I chatted a bit about how the selection of fresh stuff there is sparse, but what they do have is generally top-notch. A good rule there as at other supermarkets around here is that if it’s cheap it’s probably good. If tomatoes are $.89/lb or less there, they’re fresh and fantastic, but if bell peppers are $1.50 each they’ll be soggy crap. I held their place while they went off to experience the wonders of the produce aisle, and they both came back loaded down. Finally the line started to move with a third cashier, and we were out of there. long story, but that was how place-holding in line turned a 45-minute wait into a fun time, and an opportunity to share my produce secrets!
*semi-trashy in that it basically marks the geographical border between my neighborhood and the projects, and the food is pretty much all generic and super-duper cheap, but it is generally clean, well lit and the staff is friendly. I feel blessed, especially since it means that I can spend less money when I just run out to grab something during the week compared to my big bi-weekly trip to the somewhat more upscale joint across town. Also they always have roma tomatoes for less than $1/lb and they are usually tasty after a few days ripening in a paper bag. O luxury!