This rumor pops up every year!
Here are my observations:
First trip 2001- busses all said MGM-Studios
Second Trip 2002- busses said either MGM-Studios or Disney-MGM Studios
3rd, 4th trips 2003- busses said mainly Disney Studios
5th trip 2004- busses said either Disney-MGM Studios or MGM-Studios
6th trip 2005- busses mainly said MGM-Studios
So, who the heck knows. Every time we go, the busses say something different, but I think currently the official name is Disney MGM Studios.