There are some really cool things going on in endodontic research these days.  One of which has to do with endodontic regeneration.

An in progress case report can be found on PubMed here.

What the researchers are finding is that in young permanent teeth with necrotic pulps and periapical pathology, revascularization can actually return vitality to the tooth.  As a result the periapical pathology can resolve as well.  The thought is that stem cells around the apex of young teeth can be utilized to regenerate a dental pulp.  There is some argument as to the best procedure to do this.  In the PubMed case report I listed above, the endodontist utilized coronal irrigation with no instrumentation.  Other procedures utilize taking endodontic files out of the apex to draw stem cells back into the tooth.  MTA is of great use during these procedures as well.
There is some thought in endodontic circles that this could eventually lead to a product with growth factors that could be injected into a necrotic pulp that could return a tooth to vitality without performing endodontic treatment.  This is some ways away, but steps are being made in that direction.  A good friend of mind did research on this during his endo residency.  I will try and get some periapical radiographs of some cases he did and post them here.