Basically, I am a big fan of normalization (since none of the other ways seem elegant to me). For example, if I were going to write the schema for a Blog, I would have an articles table, a tags table, and an article<->tag mapping table. This means that "show me articles with tag foo" is indexed (and "faster" than an O(n) search over the articles table).
The only approach I can see with SimpleDB is to tolerate scanning all articles to search tags (and authors, etc.) or to maintain a separate "document" for each tag (that links to the articles with the tag). Then I worry about concurrent updates to a tag ( {foo:[1,2]} gets updated to {foo:[1,2,3]} while someone else immediately changes it to {foo:[1,2,4]}; the correct result would be {foo:[1,2,3,4]} but adding 4 overwrites adding 3.)
Anyway, like I said, I would be interested in hearing people's experiences. Perhaps I am missing something obvious.