We use Magento as our ecommerce platform, so I thought I would submit here, what I used to cut the rubbish out of the sitemap.
Exclude URLs;
limit=
mode=
order=
price=
I also have the following in my robots.txt (maybe the site admin will know if these can be added to the exclude URL's box.
Disallow: /app/
Disallow: /catalogsearch/
Disallow: /checkout/
Disallow: /customer/
Disallow: /downloader/
Disallow: /js/
Disallow: /lib/
Disallow: /media/
Disallow: /report/
Disallow: /review/
Disallow: /skin/
Disallow: /var/
Hope that helps, and that It can maybe get added in the next version.