There is a known issue with Safari and Chrome, when you can't pass a focus to plugin (Flash or Silverlight). There is also a question here at SO that asks how to solve it (marked as answered, though technically it's not the answer).
I'm really concerned with this situation, since it totally breaks user experience . Do you know somebody who develops Webkit? What can we do? Post a comment on this issue? Write a fix? Convince users to switch to FF?