I think GSDS is no longer a matter of "if" but when". Right now we're in that loop where there isn't too much new news, so an annoucement about a new game might be made in the sumer, probably around the Japanese release of movie 12.
Than again the annoucement about Mystery Dungen Sky kind of negates that theory. But I imagine that we'll see GSDS by the end of 2010, if not this year, since the 10th anniversary of GS is coming up.
Correct - the DSi is merely what the GBA SP was to the GBA, or what the DS Lite was to the DS - a mere upgrade. There won't be games specific to the DSi, I'm pretty sure - all it has are some extra functions and all, and a somewhat larger pricce tag as a result of that.
DS->DSi is more like GB ->GBC. It's nearly the same, with a few new features, not just a hardware revision.
New main series pokemon games are gonna have to be released in the DSi, because as it stands the "DS2" proper is no where in sight, and I'm pretty sure that the 5th Gen will be on the next new handgeld, wich is not the DSi.
And when I say released on the DSi I mean have DSi exclusive features, not DSi only. Like the extra features GSC had on a GBC (color, etc.)
So my point in all that was, the release of the DSi extends G4, therefore IMO further implicating GSDS.