Hey Jonathon,
I wouldn't entirely rely on creating your customers site's using HTML 5 and CSS3, however start thinking of implementing rich media i.e. flash, silverlight in HTML 5 to benefit from code bloating and improving the sites crawlability and indexation.
All browsers do render HTML 5 & CSS 3 however they have not become popular enough to become standards. I'm suspecting a move next year.
Here's an interesting article about HTML 5 & SEO if your interested- http://searchengineland.com/seo-best-practices-for-html5-truths-half-truths-outright-lies-99406
Hope this helps,
Vahe