Mirrors. Definitely mirrors. One for each blind spot on the sides and one that goes straight back. And radar, but not necessarily a grid map, just a radar that shows where the bikes are.
But the mirrors gotta have. No ifs ands or buts about that.
Yeah, hit "glance this way" buttons all day, but think about it. If you were driving your car and you had to press a button to look behind you, would you consider that acceptable UI?
The only real problem I see with mirrors is already being pointed out with the gauges, people that have them will have an advantage over people that don't have them. THere is a corollary problem with giving up screen real estate for the mirrors, but I think I can make that sacriifice, personally. I was thinking maybe a top frame in single player mode, and then either taking them away for split-screen or just having smaller mirrors.