Is there an easy way to convert a Blu-Ray image into a DVD one (shrinking the frames from 1080P to NTSC), rather than rendering twice? I'm using Encore CS4, and it takes quite a while to render an hour's worth of video.

The short answer is no. It is effectively a two step process. You'll need to extract the video from the BD, then convert it for the DVD. That conversion will take some time since it needs to down-sample the 1080p video. You might save some time by staying progressive.