Back in the early 00's, generations were very commonly referred to by names coined by the fans. The Generation I, whose main versions' naming schemes were colors was called the Color Generation. A similar logic was applied to Generation II, otherwise known as the Metal Generation. When time for a Pokémon game on GBA came -- and actually prior to the announcement of Ruby and Sapphire -- the Pokémon Advance placeholder for the then soon-to-be-announced games gave its name to the Generation III, possibly backed by the newly announced anime series too; the Advance Generation. Around that time, generations started to be referred to by just their number, and nicknames fell into obscurity.
Despite the fact that numbers were fully used at that point, in 2009 the name '3D Generation' was given to Generation IV in Bulbapedia. In a similarly shy fashion, Generation V's moniker became the Monochrome Generation.
Today, Generation VI was announced for 3DS. What shall we call Generation VI? :)
As a side note, how would you feel to see Generation IV renamed to something else after it's hardly the '3D Generation'? Not to suggest that Generation VI should necessarily be named that instead, but it's no secret that with the games being the first true conversion to 3D in the main series, with even their naming theme after the axes suggesting that, it seems a bit wrong to call Generation IV the '3D Generation'... Back in June 2012, I put that suggestion on the table, but there was no interest. So, since we have the opportunity -- could you see Gen IV's nickname changed?