Aaron at Unlocking iMovie ’08 offers some good tips for getting your iMovie video into iDVD. Since iMovie no longer includes the capability to make DVD chapter markers, a common workaround is to bring the movie into GarageBand and enter markers there before zipping it along to iDVD.
However, there appears to be problems with working on large movies losing their audio and in the quality of the video once it arrives (since by the time you get to iDVD, the video has gone through several rounds of encoding and compression). A workaround for the latter is to export the movie using Apple Intermediate Codec instead of the default H.264.
