Thanks balinor, yeah I hope it helps a few people out.
I was really disappointed when I realised that xcart didnt have this functionality - I thought it was part of the standard package.
OSCommerce has this feature as standard, and in my opinion it should be standard in xcart too.
I think it's quite a valuable feature because it allows to you track your customers as they move around.
In my experience at least, I find that most customers are unregistered until they are actually ready to buy something, at which point they will register. So for the majority of users on the site you dont really get any valuable insight into what they're doing - all you see is that 'someone' is on 'a particular page'. And if you have a lot of unregistered users on your site there is no way of keeping track of a specific user.
Whereas when you can see their ip adress, the unregistered user loses some of their anonymity, so now you can see that user-1 was on the home page, now they've gone to cat-x, then product-y etc. So you are seeing the movements of your potential customers almost as it happens.
Watching customers as they surf your site and buy (or not buy) in this way can help you lotate bottlenecks or problems in your site to help streamline the buying process that you have implemented.
Incidentally, I also added another feature to this mod which displays the previous page the customer was on before they arrived at the current page.
If anyone's interested I'll post that mod as well.