IMHO biggest threat now is:
- PPC, now that latest models have reasonable battery life and better stability. Even push email if your've got Exchange. They're not 'there' yet for those who recognise security and management as the real issues thay are for corps, but the gap is narrowing fast
- Nokia's new device (E61 is it?) with a half decent keyboard is a challenger, I think.
Note I don't think "push" is actually the big issue (some corps run BES's that are so underspecced in a poor environment that "push" is up to 10mins anyway
. It's about good hardware (physically robust, and battery life) good software (stability), and good managability (remote, for corps).