The phrase was replaced with the "Switch" and "Get A Mac" campaigns, but didn't disappear for good. A portion of the poem from the TV ad can be found on the high-res version of the TextEdit icon in Leopard and Snow Leopard. Earlier this year, "Think Different" began appearing on 21.5" and 27" iMac boxes.
This week, Patently Apple reports that the US Patent & Trademark Office has published Apple's new trademark application for Think Different (it's application 77882684 if you were wondering). It could mean a revitalization of the campaign, but it's more likely that Apple just wants to keep it in their stable. Many consumers still think of Apple when hearing "think different."
Like most iconic aspects of pop culture, there have been many parodies. Here's my favorite.