Sorry for previous message having no comments.
Just remark:
These aggregates created successfuly both in 8.2 and 8.3beta4:
CREATE AGGREGATE array_concat(anyarray) (SFUNC=array_cat,STYPE=anyarray
);
CREATE AGGREGATE array_build(anyelement) (SFUNC=array_append,STYPE=anyarray
);
But aggregate from first letter does not:
create aggregate build_group(anyelement, int4) ( SFUNC= add_group, STYPE = anyarray
);
Excuse me for being noisy and bad English.