Googler, JohnMu, said Google does index and rank .Mac files. He linked to a site command for site:web.mac.com that clearly shows Google is indexing those files. But it appears Yahoo is indexing a lot more than Google, while Live Search is about the same as Google.
So in short, search engines have no reason not to index .mac hosted content. Just make sure those pages are search engine friendly and allowed via the robots.txt file and you should be set.
Forum discussion at Google Groups.