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If you want to play them via iPhoto you're going to have to use something like ClipWrap to change the wrapper (without transcoding) from MTS to MOV, before import into iPhoto.

(I can't vouch for this personally, so test first to make sure it does actually work as expected.)

Clipwrap looks about right, even if 50$ is expensive for what I would expect Aplle software to do out of the box!
 
To import an AVCHD movie into iMovie:

1. Make sure that the files are in the original folder structure created by the camera. This does not need to be the original disk, but it does need to be in the original folder structure: /private/avchd/bdmv/stream/*.mts
* If all you have are the *.mts files, just put them in a folder structure that matches. This can be on a DVD, SD, or even a standard external FireWire/USB hard drive.
2. Open iMovie, and select "Import from Camera"
* iMovie should find the /private/avchd/... folder structure, and treat it as an AVCHD camera and allow you to import.

Thanks for a comprehensive answer. I already bumped into the folders structure thing and I don't like having my movies stored like that, they should be in the same folder as the pictures of the same subject. Too much hassle Apple! Going through iMovie sort of makes sense, even if it should not be necessary when the movies does not need editing.
Again - If I wanted tedious workarounds I would have stuck with Windows...
 
Hm, just tried the Clipwrap trial (version 2.0.1 I believe) - movie stutters and is full of artifacts - useless!
 
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