In the past Quicktime would not play an AVCHD file on my Mac.
Today I discovered now it will.

I'm not sure if it's because I switched to OS Mountain Lion, updated Quicktime or updated Final Cut X.

It's a Mountain Lion feature.

Thank you Alister.

it only plays the avchd folder. i wish MTS can be played from Preview too

If you only have mts files, then use ClipWrap to rewrap them to .movs.