OK... so what's the rumour mill saying?
I remember seeing a spy-pic of a watercooled boxer on the net - it almost seemed like the cylinders were slightly angled upwards (like a 160° v-twin). Also, Motorcyclist Magazine speculated about a 4-cylinder watercooled boxer a few years ago.
Seems its only a matter of time before BMW has to introduce water cooling. The HP-Sport has already pushed the theoretical limits of how much power a 1200cc boxer can put out.
Cheers,
Lucky
1-Find road, 2-Aim handlebars, 3-Release clutch and twist throttle, 4-Repeat