AppleInsider, quoting "people familiar with the matter" says iOS 4.1 is currently being tested by Apple not only on iPhone 4 (currently available to developers) but the upcoming iPod touch 4 refresh, iPad 2, and an unknown mystery device.
iPod touch 4 is pretty much a no-brainer at this point. For the last 3 years Apple has shipped a new iPod touch in the fall along with an x.1 software update, so we'll consider that a done deal. The iPod 4,1 hardware designation means it will also once again be a major update. Retina display and dual cameras would certainly qualify.
As to iPad 2, also a major update if iProd 2,1 is the proper designator, that might get a camera as well come next spring. However, since Apple hasn't even introduced an iOS 4.x beta for the current generation iPad, it's harder to predict what the iOS 4.x situation will be by then. 4.2 to mirror 3.2 last year?
As to the mystery device, that might just be the iOS Apple TV/iTV we keep hearing about.
Hopefully we'll find out more at Apple traditional fall special music event for iPod and iTunes in the next month or so, but let us know your best guesses below!