This is what Sean Harry said 3 years ago when Summer cancelled a Serenity event :
Quote (Sean Harry)
Sadly, Summer Glau has had to cancel her appearance at STARFURY: SERENITY COMPLETE in 2008. On Thursday she was cast in a new film, and the shooting schedule conflicts with the convention.
I hope for Summer the same is happening as we speak.
Edit : see Fermi, i used the quote system because you don't need to scroll down in this case.