Sharif Atkins, star of ER, has been taking some phone sessions to bone up on a few dialects, including Irish and Puerto Rican.
I spent several days in the ADR studio collaborating with Laurence Fishburne, dialect coaching Ryan Phillippe as he re-records his dialogue in a Dutch accent for the upcoming feature film Five Fingers (the sound from the locations where they filmed was too noisy and unusable.