It can be a search engine nightmare, if you design things wrong. What was happening was individuals were making 100% AJAX driven sites, with no way to show a given page to a friend or for search-bots to access the site. Eventually these things cleared up, and the problem has regressed. Though it is a combination of design and implementation that, when combined, will kill your site in the listings quite effectively.

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Programmers are in a race with the Universe to create bigger and better idiot-proof programs, while the Universe is trying to create bigger and better idiots. So far the Universe is winning. - Rich Cook