I'd agree with both of the previous posts. However, I think that the simpliest solution that you're looking for (since you don't wish to reduce power in code, and want your robot to drive straight as possible) lies in basically an extra pair of gears somewhere among your reductions to effectively reverse the direction of one motor. While you might loose a few percent efficency.. it's an easy thing to do.
I'm incredibly temped to CAD out a simple gearbox that'd do just this that would be "out of the box compatiable" with last year's kit. If I didn't have another couple hours of homework... I'd be crazy enough to do it.
So.. I go ASCII style G's are gears (5 G's to a gear), +'s are bearings. Make the housing out of some 1/4 plate, you could get away with 1/2 shaft or less on those gears, and then just couple them to wherever you're outputing to.
Code:
---
|D|
|M|
---
|
---+----+---
| | | |
|GGGGGGGGGG|
| | | |
---+----+---
|
|
Anyone feel like adding to my reputation for my artistic skills? Haha, just kidding. Partially.
Matt