Independently, Tom Verhoeff solved the same problem (actually, I first solved Yahtzee with no bonuses and later worked on the complete game; he started with the complete game and in fact his results allowed me to uncover a bug in my Yahtzee bonus code). He has an excellent site with statistics and an online Yahtzee Proficiency Test, as well as a Scratch and Play book.
Phil Woodward later also solved solitaire Yahtzee and got "Yahtzee: The Solution" into Chance (v. 16 #1, pp. 18-22, 2003).
jar file for Yahtzee (password required)
jar file Combinatorics classes required for Yahtzee
jar file Generic genetic algorithms
YahtzeeBonusStrategy.java, an implementation of a straightforward but low-performing Yahtzee strategy.
StrategyMaker.java, creates and tests a Yahtzee strategy based on a randomly generated array of expected scores in each category.
Game Theory talk for Senior Project
A Retrograde Approximation Algorithm for One-Player Can't Stop from CG2006.
Computer Strategies for Solitaire Yahtzee from CIG2007.
javadoc for Yahtzee classes