The International Standard Name Identifier (ISNI) is a new international standard (ISO 27729) serving a similar purpose to artist MBIDs: uniquely identifying so-called "public identities" (names, but also pseudonyms), even in case of spelling variations or different persons sharing the same name. It is also intended as a "Rosetta stone" for translating existing codes like IPI that are limited to certain domains (in the case of IPI, comprising only authors of music). More information can be found at http://www.isni.org/
MusicBrainz should be able to store ISNIs for artists, similar to the way IPIs are already. An ISNI consists of 15 decimal digits and one check character that may be a digit or an "X". E.g., "0000 0001 1023 081X" is the ISNI of Michael Jackson, King of Pop.