I've never considered Memphis a mid major. Going back to the Hardaway days (and before then), Memphis has stayed ranked. It's just that in this day and age, the talking heads at ESPN want to label them that because they don't play in the ACC, Big Ten, Big 12 or SEC.
Gonzaga is not a mid-major anymore. When you have done what they have done for 10-15 years (#1 seeds and ranking, Sweet 16/Elite 8 runs) you can't label them that. If Butler can get back on their feet they will be in that same boat. VCU is trending that way as well since 2007.
While many believe the term "mid-major" was originally coined by the sports media in order to distinguish between BCS and non-BCS conference teams, it was actually coined in 1977 (more than two decades before the development of the BCS) by Jack Kvancz, head coach of Catholic University's men's basketball team.[1]
http://en.m.wikipedia.org/wiki/Mid-major