It looks like it might already be happening in Europe.... 200k every week in slow periods is no joke.
Of course Japan exploded earlier and (penetration rate-wise) far higher.
This graph amazes me every time I see it:
http://vgchartz.com/hwlaunch.php?cons1=DS®1=Japan&cons2=PS2®2=Japan&cons3=GBA®3=Japan&weeks=312
As you can see, at the current point in the DS's life... the PS2 was the previous fastest selling machine ever in Japan, and GBA is 2nd (and also the previous fastest handheld) but the DS obliterated them... the SNES is the 3rd fastest system if you want to change one to that too btw.
for every 5 people in Japan there is 1 DS... of course many will have multiple DSs so it won't quite be 1 in 5 people own a DS, but it is certainly more than 1 in 10 people own at least 1 DS.
The UK is by a decent margin the next best, which is about 15% or 1 DS for every 7. (though again, multiple owners probably mean it's nearer 1 in 13 or 14 people owning DS.... and I am not even one of them)
Here is the UK graph... where before DS it was PS2 being the fastest again (although there is a lack of old data I doubt SNES or PS1 were faster than PS2)
http://vgchartz.com/hwlaunch.php?cons1=DS®1=UK&cons2=PS2®2=UK&cons3=Wii®3=UK&weeks=312
In the UK it looks like there will be a new contender with the Wii, which is tracking way above eve the DS at it's point in life.
I think the continually increasing gradient you see for the UK is showing what you are saying... mor people are buying DSs because more people have them, like a snowball effect.
As seen with Japan, it can't be a neverending process because of population limitations.
