My dream DBZ cast:
Goku - Robin Shou (Mortal Kombat)
Piccolo - Peter Mensah (Tears of the Sun, Hidalgo, 300)
Vegeta - Cary-Hiroyuki Tagawa (Mortal Kombat as well)
Bulma - Drew Barrymore
Krillin - Danny Devito
No, I have no clue who would be good...
Master Roshi - Really, any old Japanese guy with no hair and a handle-bar stash will work.
Tien - Edward Norton. Hands down.
Yamcha - Charlie Sheen, if this was 20 years ago. Nowadays, no clue.
Trunks - Leo Dicaprio...sorry...he's got the pretty-boy looks.
Gohan - Cameron Bright (Running Scared, X-Men)
Never gave thought to anyone else. That's just going on appearances alone...
*EDIT* Yes, I used to be a huge DBZ nerd when I was a kid. I know entirely too much about the show...