In a recent interview with Zap2It, Rebecca Rand Kirshner Sinclair, “90210″ new boss, commented about the dismissal of Dustin:

Fans were shocked when news broke that Dustin Milligan (Ethan) won’t be returning next year. Can you talk about the reasons behind cutting Ethan?
R: I honestly think Dustin is such a good actor and such a lovely person. He is incredibly smart and perceptive and I loved working with him. At the end of the year [when I joined the show], I was really encouraging a dialogue between the writers and the actors and he was somebody who took me up on that. He was so thoughtful and careful about his ideas, and I was really impressed. So in my mind, it wasn’t about not liking Dustin, but [about] the character of Ethan. I felt [Ethan] didn’t allow me the freedom that I wanted. I really wanted a range between the characters and I didn’t feel like I could do with that character what I wanted to do with one of my core people. I think Dustin is amazing and he’ll do great. I was very honored to work with him, but he won’t be back.

She also pretty much confirmed that Dustin won’t be back to wrap Ethan’s storyline:

Will Dustin ever show up in a season 2 episode to wrap up his storyline?
R: I’m not absolutely sure, but I don’t think so.