I've been out of the loop the last ten years but unless things have changed since I retired the paintable latex caulk recommended above is what I'd use.

FWIW,,,, James Hardie agrees.
http://www.jameshardie.com/homeowner/improveYourHome_resideYourHome_tipsArticles_caulking.shtml

One word of advice,,, when working with cement siding, don't wipe the excess caulk with your finger like you normally would or you'll leave a shiny spot after paint no matter how well you think you wiped it off. Let it dry and then cut the excess off with a utility knife. Time consuming but you'll end up with a better looking job.

Luck