My money is on October for an Apple TV announcement.
Well if the idea is to redefine the AppleTV as an iOS device, it would make sense to announce it with the iPod line. Especially if they're gearing up to give us OTA/WiFi iTunes library access.
(In fact, if you look at the Apple site right now, the Apple TV is lumped in under iPods already.)
As to these little screens and little cases, I don't think it's a nano - I think we're looking at the new shuffle with a pared-down, locked iOS distribution - or perhaps a 'Shuffle Store' you can get apps from and upload via iTunes. I really doubt they'll cut back on established Nano features to fit a form factor. If anything, I forsee the Nano ending up like a smaller iPod Touch.
(Edit - although, someone mentioned above the iTouch is really a better option for not much more money. Maybe broader storage options for the iTouch and do away with the current Nano form factor? That would leave this new square shuffle as the entry level. Hmm.)
When you look at things from Apple's perspective, right now they've got a bunch of OS's running concurrently that can easily be pared down to just two - OSX for your computer, and iOS for your consumer devices. I'd be surprised if the current iPod OS survives past this event; the total lack of any work done recently with the Apple TV OS suggests that's on the way out as well. It makes perfect sense for them to consolidate and present a unified experience across their CE devices.
Along with this, I think once 4.0 hits the iPad we'll see less fragmentation amongst iOS devices - the major revisions can easily hit this hypothetical iOS range at the same time, with functionality being the only real difference (ie, you're not likely to see an app store on a 3" square touch screen, AppleTV will be remote-controlled). If they're all working from the same code base, it's not going to be difficult for developers to create apps targeting a specific functionality either (Netflix or Hulu Plus for Apple TV, Nike+ integration on a shuffle/nano, etc).
tl;dr Apple will streamline CE devices to all run iOS, differentiation will be in functionality and input methods alone.