IAB/IESG Recommendations on IPv6 Address Allocations to Sites ("RFC 3177":http://tools.ietf.org/html/rfc3177) provides recommendations that for end sites a @/48@ block should be provided in normal case. A @/64@ block when it is absolutely sure that only one subnet is needed and @/128@ for the case that only one device is connected. The requirements for IPv6 in 1993 included the plan that the next IP version should address approximately 2^40^ networks and 2^50^ hosts. Therefore the currently IPv6 address can be loosely splitted in a 64 bit network number (including subneting) and 64 bit host number (including flat EUI-64 host part and randomly self autoconfigured host number) -> 2^40^ & 2^50^ goal accomplished.
@/48@ provides end sites the ability to subnet 2^16^ subnet numbers for internal routing infrastructure each with theoretical max. 2^64^ unique hosts. Most enterprises should be happy with this. Very large enterprises should be provided with a @/47@ or with multiple@/48@. "RFC 3177":http://tools.ietf.org/html/rfc3177 constitute a more or less hard default setting and Regional Internet Registries (RIRs) should bear on that. The idea of the IAB/IESG recommendation was that a hard structure will reduce among other things maintainership (e.g. address restructuring, see paragraph 3 for more information).
"IPv6 Address Assignment to End Sites":http://tools.ietf.org/html/draft-ietf-v6ops-3177bis-end-sites-01 started now to obsolete "RFC 3177":http://tools.ietf.org/html/rfc3177. Thomas Narten et. al. stated that the RIRs originally started with @/48@ but began to switch to other policies in 2005. Namely APNIC, APNIC and RIPE encourage the assignment of smaller (e.g. @/56@) blocks to end sites. One concern is that the hard suggestion can lead to a classfull routing where CIDR continues to apply to all bits of the routing prefixes. Another aspect is that RIRs may have other policies which fit better in their model.