The fundamentals for ranking is more or less the same, keywords, content, etc. From experience, ranking well on Goolge doesnt ensure you rank well on Yahoo. There's a slight difference with ranking algo for Google and Yahoo. For one some sites that provide link juice on google might not on yahoo.
With Yahoo! commanding approximately 18% of the total U.S. search as of March 2008 according to Nielson Netratings, then yes, it is still worth optimizing a site for Yahoo!
Concentrate on GOOGLE (78% OF US SEARCHES)..... and Yahoo will eventually come around. I noticed that Yahoo seems to count links more than Google Conclusion: sites with lots of links do better in Yahoo search? (need more data to verify)