Jon,
You post just about everywhere with your checkout mod ... It's clean and I like it, but this post is about X-Cart's look of the cart.tpl, which isn't the end-all of the shopping experience for the customer.
X-Cart, or your mod for that matter, doesn't fix the fact that the products are displayed vertically. For an order of 1-3 items, this works. For orders with 10 items, having a cart that looks like a stack of product.tpl's is too much!
Another thing that X-Cart (and your mod) should do is to shorten the pages of "cart to checkout". I mean, why even have a "cart.php" page with the format of elab's image? That's a page that should be dropped from the XC system entirely (1 page - that's huge!). The customer should view cart at:
cart.php?mode=checkout which is the 1st page of the checkout. This is what nearly EVERY mainstream store has and some even have small icons.
The plus of skipping the one page, is to go with a horizontal viewing of the cart. A shopping list versus a small basket.. Now saying all of this and it doesn't really have to do with you or your mod, though you do push it quite a bit

- is to change the way a customer views the cart. All one would have to do is skip the "cart.php" page and go straight to the "cart.php?mode=checkout".
Now that is a solid solution that I just thought of while looking at the original post. All one would have to do is go to the /customer/main/minicart.tpl and change the "View Cart" link to the one that is used for "Checkout". You could even put in some product icons/images.
Ideas on this? Seems pretty reasonable to me.
- Mike