I have several database driven sites and do use query strings (with one variable) and the sites all get indexed and rank well.

The only advice I'd give is that if you want ALL your pages indexed, make sure they're ALL listed in a sitemap at the root level or linked from your main pages.

I have two sites with several thousand records, and thus several thousand dynamic pages, and a user has to do a search to get to some of the older content. The new content is all linked from the main pages and the search engines index it fine - but some of the older content doesn't get found. That works for my site just fine, but your situation may be different.