For FLAC to MP3 conversion there is flac2mp3.pl, a perl-script which maintains a parallel MP3-library. It traverses the FLAC-library structure and transcodes any new files to the corresponding MP3-library. Updated tags on files will also be spotted and transferred, without transcoding if the checksum for the sound content is unchanged. Just launch this script whenever you have added new FLAC files or updated tags.For some reason Robin Bowes' flac2mp3.pl site is down at the moment. There you will find more information.
The released version also doesn't copy non-flac files (e.g. mp3 or jpg-files) in the FLAC library structure over to the MP3-library but I have made a modified version which handles that too. If you are interested PM me here on Hydrogenaudio and I will send you my version.