Surely an oversight

http://musicbrainz.org/ws/2/release?recording=4c70d157-6a4c-4d34-9970-61a3721f9542&inc=labels+artist-credits&type=album

would expect to include

<release-group type="Album"/>

and from http://wiki.musicbrainz.org/XMLWebService

Any query which includes release-groups in the results can be filtered to only include release groups of a certain type. Any query which includes releases in the results can be filtered to only include releases of a certain type and/or status