My company's web-site is all about online games, so accessibility is not high on our priority list. SEO best practices, however, are. Searching on the net we couldn't find any discussion of whether or not ARIA is such a best practice (which is a kind of answer already :-). I found this surprising because using ARIA roles seems like a natural thing to do - they contain a lot of SEO-relevant meta-data (overall page structure, what parts of the page are the "main" as opposed to "service" navigation area, what parts contain "actual" vs. "related" vs. "independent" content, etc.). What's more, given they effect the user interface (screen readers and so on) they would tend to be pretty accurate when they exist.
Does anyone have specific knowledge about whether any search engines actually use this data, if it exists in a page?