This drove me nuts too - you can export from iMovie, then try to import back to combine projects - and it refuses - greys out the file in Finder.
The Chapter markers are indeed the key.
The fastest solution is to use Quicktime7 (not any later versions - it's the last one which had editing/cut/paste functions and is still available for download if you google it).
In QT7, open the mov file.
Click Window / Show Movie Properties
This shows the components of the file. There should be a name entry, a video track and a sound track ONLY.
If there is a chapter track, or other text track, click it, hit delete.
Click File/Save (or Save As if you want to leave the original file intact).
Now, back to iMovie09, and the newly saved file should import OK.
Of course, you now need to reinstate the chapter markers...