Final Cut Express support .MTS files or not ?

The format is not currently supported by any application on the Mac.

H.264 is a delivery format using interframe compression. It's very difficult to work with in editing applications without doing a file conversion. It's also a highly compressed format, only 15Mbps compared to HDV and DV's 25Mbps, which means conversion is problematic.

As FCE is heavily dependent on Final Cut Pro, basically a cut down version, it would probably depend on FCP incorporating the format. As it's a format unlikely to appeal to professional video production, it's unlikely to get incorporated into that product.

It's possible iMovie will incorporate in a later release, in which case it might migrate to FCE, just as HDV was incorporated into FCE and iMovie before it was incorporated into FCP. But that'll probably depend on how popular the format becomes. It will also depend on how easily the Mac platform can read the hard drives and formatting used by these cameras.