[greencactus3]

do you mean as a "square robot" with its wheels on 2 diagonal corners and 2 on the other 2 corners? will this robot drive straight like that or will it drive with one caster "point" as forwards?
if you are going for a diamond sahped robot thats cool. but its still a center 2wd so itll be very manueverable as in very turny. but harder to go straight than a 'normal' 2wd and if you mean as in

PW=powered wheel
CW = caster

....(forwards)
PW----------CW
|....................|
|....................|
|....................|
|....................|
|....................|
CW----------PW


then this robot will turn in the left direction much better than in the right direction. this i think will be hard to drive too. as the pivot point in a one side braked-and-other-side-powered-turn different in each direction. i see no reason to do this.

................CW
............../......\
............/..........\
........../..............\
.......PW..............PW
..........\.............../
............\............/
..............\........./
................CW
would be a fine design i think.
it would be a wider stance than most robots. but will sacrifice a bit of foot print as to make it work well you'd want to make it a all same sided square and the rule's dimension is not like that.