It seems a bit presumptuous - perhaps greedy, even - to be wondering about the DS2 when we've only just been treated to the DSi, but in a year that Sony will release a PSP upgrade, and where the iPhone is perhaps starting to impinge on its territory, will Nintendo be working on an upgraded version as we speak?
I think it's a safe bet to say 'yes'. Recently, Nintendo funded the construction of a new campus for an additional R & D department said to be specialising in the development of new hardware. Though the DS2 will likely have been in the embryonic stage of planning at the very least already. As a format holder Nintendo very much likes to plan ahead - indeed, the DS itself was originally planned as a third pillar around the time of the GameBoy Advance, and it was only when it started seeing unprecedented levels of success that Nintendo decided it should be a direct successor to the GBA. It's fair to assume that the GameBoy name, therefore, is effectively dead - unthinkable just a few short years ago.
With DS now becoming a household name, it's therefore much more likely that any new handheld will be called the DS2, or something similar. It's hard to imagine any redesign of the hardware being too radical, although I wouldn't be surprised to see Nintendo go down the digital-download route for software, like the iPhone and PSPGo - particularly given that it almost certainly won't be released for a good couple of years at the very least.
But then again, Nintendo hasn't embraced online in anything like the way Apple and Sony have, being reluctant to really commit to downloadable content in a meaningful way besides WiiWare and DSiWare, and even those seem like half-hearted sops to the relatively low percentage of people who regularly take their Nintendo consoles online. Perhaps as the prevalence of broadband increases and audiences become a little more tech-savvy, then the download model will be the way forward for Nintendo - it's certainly not done any harm to the iPhone.
I'd also expect both screens to support touchscreen functionality, while tilt-based control wouldn't be out of the question, either - it might be a little awkward for twin-screened games, but perhaps one of the screens could tuck away neatly if not in play. The button layout would likely remain the same, though with some audiences clamouring for a portable console with the power of a GameCube, maybe a couple of analogue nubs could find their way onto the base.
At this stage, it's little more than speculation, though. It'll take a pretty seismic shift in the portable market for Nintendo to even consider launching a new DS in the next couple of years, so unless Apple starts to seriously eat into Nintendo's market share (sorry, Sony, but I don't think you've got a chance) then another hardware revision rather than a brand new console is the most likely upgrade the DS will get in the forseeable future.