supported codecs

First of all:
iMovie is no 'all purpose'-editor.
It is meant by concept for camcorder-imports!

mov (or avi) is NO format, just a media-container or 'wrapper'!
just because the Quicktime-player is able to plaay it, does NOT mean, iM is able to handle it.

List of iMovie09-supported codecs, not tested of completeness:

Apple Intermediate Codec
MPEG-4 (some .. )
H.264 (some .. )
Apple Animation
Apple Video
ProRes422 (if Final Cut Pro is installed on your Mac)
AVCHD/AVCHD-lite IF imported from Camera or from an iM09-generated 'archive'.
Mpeg2 IF imported from Camera or from an iM09-generated 'archive'.

AVCHD and mp2 will be automatically on import converted to appleintermediate.

iMovie09 will not accept files containing extraneous data-tracks such as (thanks to appleman1958 for this info):
Closed Caption
Secondary audio such as AC3

iMovie09 does not support, although identified with suffix .mp4, DivX or XviD.

wmv is not supported.

QT-plugins as can teach the underlying QT-engine new formats, that does NOT mean, it necessarily is handled by iMovie.

For stills and audio, rule-of-thumb is, any Quicktime-supported format can be imported, although jpeg, tiff, png and aac, aiff, 48kHz is preferred.

avoid Mac-exotic formats as ogg or mkv-containers..

there are lots of converter-tools avail, which allow to convert many 'videos' into an iM-acceptable codec.