wait didnt we think of the same thing with brock? when he left and it was only misty, ash and tracy. and we thought that he was gone forever. well maybe it's the same case with misty. maybe the creators are biding there time for the right moment to put misty in =D
When's the "right moment"? The way I see it, the writers have had multiple chances of having Misty back into the group, but they didn't keep her in it. Since they make the series, they have the power to make Misty come back.
Also, wasn't Tracey unpopular with people, causing Brock to have to come back? In Misty's case, May was popular and I'm sure that Dawn's real popular too. (The producers would have made sure of that.)
Anyway, I'm sure the thing that Kasumi's VA mentioned had already passed, but I'm also sure that Misty'll come back, just not forever.