Wowza 3 Transcoder does not support decode of Spark video. ffmpeg can work, but the results were marginal, to be kind, in my tests.
The only good option right now is Flash 11 h.264/Speex > Wowza Transcoder > h.264/AAC. It's easy enough to require a minimum version Flash plugin for your application.
ffmpeg -i rtmp://localhost:1935/videochat/testing -re -strict experimental -acodec aac -ar 22050 -vcodec libx264 -vpre default -vpre ipod320 -threads 64 -f flv rtmp://localhost/live/testing_mp4