It's quite true that it's been possible to get a SIP card giving access to the same health care as Spanish nationals since August 2012. We're also under state retirement age and went through the procedure last October with no problem.
The documents you'll need are:
valid passport
green certificate showing you are on the Foreigners' Register which must be dated before 24 April 2012
letter confirming you have no social security cover in your country of origin (obtained by calling the DWP in Newcastle)
Document to show your income; the regulations state it must be under 100,000 euros/year (this may not be essential, we weren't asked for proof but had taken our Renta declarations in case)
Padron certificate
The procedure in the Valencian Community is to make an appointment with the local social security office by phone or online although it can't be done online using a Mac. You present all original documents except the Padron, plus photocopies of passport, certificate & letter. In return you're given a form called Documento Acreditativo del Derecho a Assistencia Sanitaria which has to be taken to your local medical centre, with a Padron certificate and ID, and a SIP card will be issued.