i'm with epopuI on the kensington, it's great. sturdy enough, easy to clean (ball isn't locked, just turn it upside down at catch) when needed, and it's comfy for a center-ball trackball. supports macros and the like out of the box too, i think (not using it right now, mom stole it). four buttons and a (non-clickable) scrollwheel.
armrest is detachable, i use it without.
searching other kensington trackballs, it seems they may have some other ones you might like.
they all are pricey though, and if you have young kids they will play with the ball.
[url=http://www.mousetrak.com/products/evolution/evo.html]this one looked interesting, though i have no idea how available it is.[\url]