It started development in 1986, but didn't finish development to be released into a product until 1994, five years after the release of the GameBoy.

Edit: Also Michael Johas Teener, the one time chair of the FireWire working group, says here in slides from a presentation about the history of firewire, hosted on his personal website, that the "Connector based on Nintendo Gameboy". http://www.johasteener.com/docs_files/1394HistoryAndMarket.p...

There's some what would be hilarious if it wasn't so sad commentary on the GameBoy Link Cable wiki talk page about how that isn't enough of a reference for some reason. The deletionists have certainly won wiki.