Beware if you use the unique user information to id the cart is in the URL. My dynamic cart software did that with "usr=12345678".

G indexed unique pages with the "usr" parameter. And, each time G visited my site, "new" pages showed up because a different "usr" parameter was assigned. The pages were the same, but the "usr=" parameter was different.

Duplicate page penalty? I had a hard time getting indexed in G. I thought it was Big Daddy, but now I think it was several reasons, the innocent duplicate pages being one reason why.

If someone clicked that link in the G search results, then they would see that usr's cart! Not good.

G recommends to use "id=" for the unique cart parameter. That meant I would have to hack the software to comply and change "usr=" to "id=". What a pita.

What if the other search engines don't follow the "id=" convention?

I don't know what the other SE's do and was getting REALLY frustrated having my site design and configuration controlled by the SE and discovering the right way via trial and error and loss of sales.

I eventually changed the whole software to use search-engine friendly URLs using mod_rewrite. A real pita.

When I first converted from static to dynamic site, the site PR dropped from 4 to 0 in G but apparently fares okay in the other SE's. It's still at zero for no apparent reason. Sigh.

Just be careful and get alot of expert and experienced advice. If you change something then wait to see results, you're losing money.

Good luck! Dynamic sites are the way to go if the SE's can keep up with technology and stop penalizing the innocents.