We've debated putting thumbnail shots in our cart, but haven't done so yet.
Some arguments in favor of cart thumbs:
1) Use them in cart if customers tend to order the wrong item as demonstrated by a high level of follow up phone calls and emails and especially product returns and even chargebacks.
2) Use thumbs when text description is inadequate and photo is more appropriate (ie. apparel, wall paper)
3) The big boys use cart thumbs and little sites often don't. Makes your operation look professional.
4) May be helpful when customers often order a long shopping list of products, not just one or two.
Some arguments against cart thumbs:
1) If a product comes in many colors (ie. tee shirt), will thumb in different color just add to confusion? Do you have the ability to serve up picture of every variation?
2) Avoid using thumbs that can't show key product details. (ie. lookalike computer chips)
3) If your cart is already cluttered as many are, don't add thumbs just to have the latest fad bell/whistle.
4) I've never seen a study suggesting that adding cart thumbs increases a site's Bottom Line.
So which way to go?
We almost never have a customer complain that what he received was different than what he thought he ordered. Our excellent product graphics and text description BEFORE the cart seem sufficient. No thumbs for us... so far.