I have been following the with interest for the past 5 years!! Well done Apple!!!
This is a joke - i bought my iMac in 2009 with a view to editing high quality home video HD footage and after MANY enquiries with Apple Tech Support was given the resounding disclosure that "iMovie is just a toy bundled for free.... if you want something better then you need a better program!!!".
Sp here we are, another update from Apple saying that AVCHD is now supported (albeit much later than Windows based products which I had to resort using through bootcamp on the iMac) and still now it exports to MP4!!! Will this ever be something good?
Though the new iMovie 10.0 for Mac OS X Mavericks/Yosemite claims the native support for AVCHD, its support for AVCHD is just the same as iMovie 11: iMovie can import and edit native AVCHD in 1080i 50/60, 720p 24/25/30, 1080p 24/25/30, but iMovie does not support AVCHD 1080p 50/60 or 720p 60.
That is, you will still fail to import AVCHD to iMovie 10/11/09/08/HD if the AVCHD footage recorded at 1080/60p, 1080/50p or 720/60p with whatever model of cameras or camcorders like Panasonic Lumix DMC, Panasonic HDC series, Canon XA20, Pana TZ7, Sony Alpha DSLR(SLT) Cameras, Sony HDR-PJ790 and etc.
In order to successfully import all kinds of AVCHD to iMovie for Mac or iOS, a recommended solution is convert AVCHD to iMovie more supported video format.