rdf(s) decidablity?

In [1] section 3.1: 
   "although RDFS seems to be a very simple language (much too simple
for many Semantic Web applications), its transformation into FOL is not
included in any of the above mentioned decidable fragments."
 
Is this mean that RDFS is undecidable? Is there any formal proof?
 
As regards to RDF Model Theory, is there any complexity results about
rdfs entailment?
 
Thanks!
 
Liu shengping
 
[1] Ian Horrocks, Peter F. Patel-Schneider , Three Theses of
Representation in the Semantic Web, WWW2003.