Apple will most likely preview iOS 5 for iPhone, iPad, and iPod touch in early April. This isn't based on a "source familiar with the matter" but on simple history. Apple is secretive but cyclical. When it comes to iOS, so far they've held to the same pattern year after year. They did a preview in April of last year for iOS 4, and they've previewed iOS (formerly iPhone OS) in the spring going back to 2.0 in 2008.
If you're curious what we hope (not expect) to see included, check out our Imagining iOS 5 post. Otherwise, we're wait
So yeah, we expect an iOS 5 preview in April, don't you?