Yes, we have seen Maleficient at both MK and Disney MGM. Check with Guest Relations upon entering the park and they can tell you when she is scheduled to appear and where.
She is a very unusual one to find greeting, but fortunately she does get out sometimes! A great place to see her is Mickey's Not-So-Scary Halloween parties, as she was in front of the Castle when we were there in 2003 and 2004. I also once saw her in Fantasyland in Magic Kingdom at the Character Festival spot, but that location has closed now.
You can also ask at Guest Services if she is out, though I wonder if sometimes they don't have much information on the obscure characters. I once asked for Brer Bear and they told me he never makes appearances, and then one minute later I ran into him!
We only saw her at MK at MNSSHP 2003 in front of the castle with all the other villains! Being westerners, we have never seen her before at DL, so it was a great treat for us!!
She's definetly rare!!! However, not 100% impossible so it could be a fun quest for you.
We really wanted to find her during our second MNSSHP this past year-- however, we didn't. Could be reason to attend the 2005 MNSSHP .
We just recently saw her at MGM at the front of the park. We have also seen her by the hat, and at MK we have seen her and the Evil Queen kind of across from Winnie the Pooh ride.
We came across her as we were exiting the Character greeting area in Toontown (forget what it's called). She was awesome! Very intimidating for my 4 year old. We got pics and autographs.
Didn't realize that she was a "rare Pokemon" as my boys would say.
We struck out looking for Maleficent during Thanksgiving week. My DD16 loves the villains and Maleficent is her absolute favorite. We'll try again on our next trip (June 2006) DD's Graduation trip. Hopefully we'll see her then.