After several attempts, I was finally able to successfully convert using VisualHub for Mac a video file for viewing on a Sony Ericsson phone.

Here’s the config:

(Convert) To: MP4
320 Pixel Wide : Checked
Quality: Standard
RTSP Streaming: Do not check
H.264 Streaming: Do not check

In the Advanced settings, check Force and select QuickTime Decoding. Ignore all the other settings.

Thanks to this article for giving me some ideas on the correct settings.