read my previous post, post 122. Of course this is all speculative, but given Beths history of trying to push hardware to their limits whenever a new TES game is released, i'd be very surprised if they didn't do it this time around, especially considering the advances that have been made in the PC market since Oblivion's release. Todd has mentioned in the past that his vision for game immersion is "realism", referring to cutting-edge graphics.
While it is likely that changes in technology and changes in the size of Bethesda will increase or decrease the amount of time it takes Bethesda to make, I still think it's a safe bet to use their past release schedule for speculating. Of course, you're right, everything we say here is just speculation based on little scraps of information like trademarks, previous development cycles and speculated console lifetimes.
One thing I feel very sure about is that there will be an announcement this year. In the last decade, we've never had a year where we didn't know something about what's going on at Bethesda in regards to TES and Fallout (not including published titles like Fallout: New Vegas). When Morrowind was released, the year after that we didn't know about their next game, but we did know they were making DLC. Then the year after that Oblivion was announced. After Oblivion was released, we knew they were making Oblivion DLC and developing Fallout 3. When Fallout 3 was released, the year after that we knew they were making Fallout 3 DLC. Not a year has passed in the last decade where we didn't know anything about what Bethesda is doing, and if they don't announce something this year, it would be genuinely shocking.