Even for adults, side impacts cause the most fatalities to properly restrained passengers. Though frontal crashes are more common and severe, the majority of fatalities are to unrestrained and improperly restrained passengers. Occupant protection systems, including 3-point seatbelts, airbags, energy absorbing dash/wheel and child seats do a very, very good job when used correctly.
As Julie said, in the past the design emphasis has been for frontal crashes. Now that fatalities in frontal crashes have been reduced and those from side impacts are becoming more prevalent to properly restrained passengers, the automakers and child seat makers have been placing more emphasis there.
For properly restrained kids above 1 year and 20 pounds, we've always known the added risks of going front facing drop off (though they never go to zero). But being in the center doesn't necessarily keep a properly restrained child any safer than being outboard. There haven't been any compelling statistics to show this. Some CHOP studies have even shown that since you never known which side will be hit, the added protection of being hit on the far side almost balances the added risk of being struck on the near side, effectively making the risks similar to being in the center. Other studies you can find say the center or outboard is safer, but none really address the question only to properly restrained children.
Just from the physics of being rear-facing, I'm keeping DS that way as long as possible. I'm not concerned about him being in the outboard seat, though.