This article say we should avoid using this technique. This one says it\'s awesome. Is it true that Google looks inside CSS files for text-indent: -9999px; and
I've read that Google doesn't promote the concept of using a negatve text-indent on anchor tags - http://maileohye.com/html-text-indent-not-messing-up-your-rankings/. Ideally, you should be using title tags and rel attributes to tell search engines more about the links.
Also, you might want to read these threads:
http://www.google.com/support/forum/p/Webmasters/thread?tid=1a51b7310162d1aa&hl=en
http://websitedesigningtips.com/5-tips-web-designers-developers-optimize-site-search-engines/ (the last point)
Sitemaps, robots, anchor tags using rel and title tags where possible and applying alt tags to your images should help better the SEO.
Google also traverses AJAX driven content now using the shebang (#!) operator so maybe that's something that you'd want to read up on - http://www.seomoz.org/blog/how-to-allow-google-to-crawl-ajax-content
Using HTML5 and JQuery, there are a number of plugins that allow bookmarking and deep linking of Ajax links.
Hope this helps.