This study will assess the immunogenicity and safety of GlaxoSmithKline (GSK) Biologicals' (formerly SB Biologicals') DTPa-HBV-IPV/Hib (Infanrix hexa™) vaccine compared with separate administration of DTPa-HBV-IPV (Infanrix penta™) and Hib (Hiberix™) vaccine administered at 3, 5 and 11 (or 12) months of age.