According their August 1, 2006 press release the Nature academic and society journals will be opening access to all online content published before January 2003. From January 2007, site license access to any NPG publication will include content from the current year plus a four year rolling archive. The exception to this rule are those journals which already offer an open archive after 12 months.
Nature journals will not have an open archive. Starting in 2007 current subscribers to Nature and it's siblings such as Nature Medicine will have content for the current year and a four year rolling archive. Archive content (older than the recent 4 years) will be available to purchase. HOWEVER, site license subscribers prior to 2006 will have post cancellation rights to all the current licensed content (e.g. back to 1997).

So there you have it, clear as mud. If it is a society journal published by NPG you might archival access after 12 months or 4 years depending on the journal. If it is a Nature journal then you don't get any free archival access.

Personally, I would just like for Nature to list on their site which journals have archival access after 12 months and which have it after 4 years. That would make my life a little easier and quite possibly theirs too because you know they are always going to get questions from people as to why this journal isn't available and that one is.