We’re getting a number of reports from Rhythmbox, Sound Juicer and other applications that their CD lookup is no longer working. We currently have no open bugs that tell us what is wrong, so we can’t fix anything.
We believe that we are outputting correct XML for CD lookups, thus there isn’t anything for us to fix. We are, however, outputting more matches for CDs, since multiple releases for a single CD can be returned. But, this is nothing new — clients should’ve supported this even before NGS.
We need the authors of programs that are broken to tell us what broke their stuff. Then we can determine if we need to fix something, the authors need to fix something or if we can come up with some sort of compromise. We’ve been talking about NGS for two years and these application authors never bothered to test their applications with our new stuff.
End users who are upset at CD Lookups failing: Please go complain to your software authors to resolve these issues or file bug reports with us if the trouble is in MusicBrainz.
NOTE: These applications are not failing because of User-Agent blocking — we haven’t actually started doing that.
MetaBrainz Blog 
I believe both those apps use the old (v1.0?) MusicBrainz API.
Gabriel: Thats fine, as long as they are not using the RDF API that we deprecated 2 years ago.
For Sound Juicer, it seems this is fixed by moving to libmusicbrainz3 (see Launchpad bug #788921).
For those running Ubuntu, 11.04 (Natty, non-LTS) seems to work and 10.10 (Maverick, non-LTS) probably does too, but 10.04 (Lucid, LTS) is definitely affected.
BTW, I realise this is a little late – I’ve just been pointed here by someone who’s affected, so maybe some end-users might find it helpful!
I don’t quite follow what formats the different versions of libmusicbrainz work with – I’m just curious really, but was this due to a change in the output, or the withdrawal of support for older, deprecated formats?
Hi Stuart – In May MusicBrainz released NGS, the so-called Next Generation Schema which apart from being a pretty fundamental database change to introduce recordings and works, genuine shared tracklists and support for multi-disc releases … was a rewrite of basically everything. 🙂
libmusicbrainz2 used an ancient RDF-based service which was long deprecated and signposted for removal 2 years ago: http://blog.musicbrainz.org/?p=392
libmusicbrainz3 uses /ws/1, the first incarnation of the XML-based web service. This service was rewritten to fit with NGS in as best a way as possible, for backwards compatibility. It still has issues with multi-disc releases since they are stored quite differently in NGS.
libmusicbrainz4 is currently in development, and uses /ws/2, the new NGS-specific web service.