The simple fact that a shopper has taken the time to add items to a cart, then for whatever reason, chooses to end their shopping session, might indicate several things.
1) High shipping prices or long delivery times
2) Comparison shopping and browsing
3) Changed Mind
4) Total cost of items is too high
5) Checkout process is too long
6) Checkout requires too much personal information
7) Site requires registration before purchase
8) Site is unstable or unreliable
9) Checkout process is confusing
There's
a really good article on cart abandonment at WebSiteGear that will no doubt be of great use to you.
Basically, it boils down to this:
If they're coming to your site and not buying, it's a lost sale. You need to know why they're leaving.