Permalinks and SEO

I've been playing with different permalink structures on my site. For the last couple of weeks I've been using /category/title. Supposedly this is the most search engine optimized, however, it frustrates me greatly since the category of a post can and does change. I'm going to settle on /year/month/title. This is more stable and will be simpler to manage in the archive. Some links may be sketchy -- I'll get them fixed over time. Google is probably pretty upset with me right now, but my sitemap is updated and correct.