By these guidelines plus
TIP #47: Package Maturity Standards, this criterion is changing. All packages go in /packages, even if they only support one database. A package cannot be maturity level 3, Mature and Standard, unless it supports both databases. All Core and Standard and .LRN packages should be at Maturity Level 3.
Jade: yes, we should roll contrib over after we ship 5.2 with working maturity level.