I think a Fall release of iOS5 has the ring of truth.
The iPod music event is becoming a bit of a non event as traditional iPod's become less relevant. So instead Apple will introduce iOS5 which will include one big music feature: cloud streaming and storage of your music!
But if you're going to introduce cloud streaming and sync, then the current iPod touch is not a great device because it has no persistent internet connection, so they're going to introduce an iPod Touch with 3G, which (obviously) won't be called the iPod Touch, but will instead be a cheaper, thinner, lighter iPhone! It might even only have 4GB of storage (probably 8GB) as standard, but the cloud services will make that less of an issue.
The iPhone 5 will still be introduced in the summer with iOS4x and will remain the high end iPhone with NFC, 1080p video recording, 4inch Screen, 8mp still camera, A5 chip, 64GB storage option, maybe they'll introduce Facetime over 3G too at that time too... Same basic physical package as the iPhone 4.
Apple needs to make a cheaper iPhone to compete with Android. For too many people now trading up from feature phones to their first smartphone (at least in the UK) the leap from what they're paying now up to an iPhone is too big. Instead they're getting a low end Android device (e.g. HTC wildfire etc). Apple are not stupid, they know they need to get into that part of the market and soon. It seems like the right time frame. And they need a big event (like their traditional fall music event) to do it.