Hi, may I suggest to support the open source AAC/MP4 encoder and decoder, too? Main advantages over QuickTime would be VBR and multichannel encoding with FAAC (especially interesting for AC3 -> AAC/MP4 transcoding) and HE AAC, Parametric Stereo and multichannel decoding as well as AAC file extension support with FAAD2.
The homepage of the FAAC project with forum and Wiki is located here:
Other useful MPEG-4 add-ons would be the existing video codecs like XviD and the MPEG-4 Systems player/muxer GPAC, also an open source project on SourceForge. There is also source code available from 3GPP for AMR decoders and encoders, so it wouldn't be necessary to buy the Pro version of QuickTime for *.3gp encoding if you include them, too.