My predictions, because it’s fairly obvious that nobody else will be making predictions about it.
(thought: Should I check the wider internet to confirm that before hitting post? Nah, I’m probably right.)
limited multitasking
notification revamp
unified mailbox support
support for faster processor, higher resolutions (but probably no specifics on upcoming hardware revisions)
iBooks for iPhone with web-based sync to iPad (possibly tied into MobileMe). Available on App Store.
some minor UI overhauling, to closer align iPhone/iPad
longshot: some kind of iChat variant with front-facing camera, built on the new notifications/multitasking changes
Apple iPhone OS 4 Revealed on April 8 - iPhone - Gizmodo
Notes