It's so difficult to predict how Nintendo will handle their next-gen plans. We hear rumors about Tegra, see patents about vibrating/pressure senstive touch screens and we have Iwata basically saying it'll have (more than just) a hi-res screen and tilt built-in. Then some look at the competition and start to think "Nintendo needs to do something soon". More telling then all the above is the DS's line-up of software and the longevity there of...which is why there's so much heat over the next PokeMon game and whether or not it'll be on the current or next DS. Then there's the whole "what about the DSi XL" people who may (price) or may not (GC micro) have a good point.
Now we're getting into the hows/whens of Nintendo revealing/launching the next DS. I think we'll know more depending upon the rumors/news made around GDC and not just about Nintendo, but about Sony & Apple plans as well. If Sony plans to unveil a "PSP2" at E3, Nintendo should be there too, but if not, then they may hold off on the reveal and only make a slight (cryptic, like the pre-DS reveal details) announcement about the "future" with a full on reveal happening later (like TGS 2010 or Nintendo Media Summit October 2010 (or Febuary 2011?) or maybe even as far as E3 2011).
The thing to consider about all this is pricing. If they truly are making a big leap with the next DS then will it be that much more expensive than the DSi XL's $189 (US) price? Or would it come after the DS lite, DSi & DSi XL all drop in price? Depends on said leap, the launch time frame and it's overall possitioning...meaning, if it's to be a console/portable hybrid, then maybe a launch price matching the Wii's ($249 (US)) price at launch would be more justified? Otherwise the price should be around $199 (US) with the current DS's all dropping in price...but with Nintendo not wanting to lower prices and arguably (along with Sony/Apple) raising the prices on portables what then??? This all leads me more to further believe that the next DS would be a hybrid portable/console to justify it's higher price above the current DS's and even the current Wii.
As far as releasing a "Wii HD" (smaller, current Wii with more storage & HDMI output)...they no longer would have to with such a hyrbid DS in place...two birds with one stone! That's not to say (as the current Wii winds down) that another next-gen Wii console-only to replace it wouldn't come, just that this hybrid DS (in some ways) could help Nintendo extend the life of the current Wii (or become it's own thing) until then.