Update 2013-12-02: I no longer use this method. See this post for my new method.
I have three (someday to be four) Roku players at home. I primarily use them to stream Netflix and Amazon Instant Video and to stream MP4s from a home server using Roksbox.
My typical workflow is to rip DVDs to VOBs using Dvdfab, copy the VOBs to my home server, and then convert them in bulk from VOB to MP4 using ffmpeg.
The Roku does not support all profiles of H.264, so here is the ffmpeg 1.0.6 command I use to create Roku-compatible MP4s:
ffmpeg -i src.vob -vcodec libx264 -threads 0 -f mp4 -y -acodec libfaac -ab 128k -ac 2 -preset slow -profile:v high -level 4 -crf 20 dest.mp4