After taking a .fo generated from DocBook XSL, there were some unresolved id's. It seems to me that fop doesn't support id= in other elements then fo:block, although it seems to me that it can appear in other elements as well.
*** This bug has been marked as a duplicate of 4652 ***
Errorneously marked as duplicate. Referring to an id of a page-sequence still results in a NullPointerException.
Correction: page-number-citation referring to page-sequence ids are still empty.
*** Bug 1231 has been marked as a duplicate of this bug. ***
*** Bug 6455 has been marked as a duplicate of this bug. ***
*** Bug 7146 has been marked as a duplicate of this bug. ***
*** Bug 18576 has been marked as a duplicate of this bug. ***
This is fixed in FOP 0.91beta but on the other side this release doesn't support "id" attributes on all other FOs, yet. For a list of those FOs that don't support the "id" attribute, please checkout the release notes at: http://xmlgraphics.apache.org/fop/relnotes.html#FOP+0.91+beta
batch transition pre-FOP1.0 resolved+fixed bugs to closed+fixed