The DSi is the newest one though, which means that's the one Nintendo wants people to buy. You never see ads for the Lite anymore, just for the DSi.
And what would that mean for people who only have a DSi?
It's interesting to see whether they will do it or not - IMO it's up in the air - could go either way.
There's the fact that there is the DSi, but on the flip side, a large number of people either have a DS, DSLite, or one of these AND a DSi. Pal Park would still be useful to many (considering the multi-million sales D/P/Pt got, at any rate). Didn't stop Platinum from getting Pal Park in it too, despite it being released in places which will get/got it after they got the DSi.
They're promoting the DSi all right, yet in ads for games they do mention the DS line (not the DSi itself) as being a console the game can be played on (leastways here it is the case). The DSi also isn't its own console in its own right, but merely an upgrade of the DS Lite - like what the Lite is to the normal DS, or the GBA SP was to GBA. Both still play basically the same games as each other (bar a Guitar Hero game, but that game is the exception to this, rather than the norm).
If they did put the Pal Park in, one would wonder where it would be though - it's not exactly the smallest place, and they'd have to put it somewhere. (Heh - if anyone remembers the guy in Kanto using a Machop to flatten the land to build something... maybe he ends up building that, finally?). One other idea that comes to mind is that Bill makes it. It does kinda fit in - in GSC, he made the Trade machine that allowed you to trade to past games in RBY. But that wouldn't be in it now - unless Bill now makes Pal Park or something like that so one can trade Pokemon from the 3rd gen games.
Considering that in Platinum, Bebe mentions Bill, and then someone who lives in Johto and had Eevee and gave them to you (which, surprise surprise, was Bill in GSC)...I wouldn't be too surprised to see him do something involving Pal Park then, or at least be in Johto. But of course this is going off random 'hints' and spur-of-the-moment speculation - first, we'll see if Pal Park is in or not. =)
As for people with only a DSi...well, Pal Park wouldn't exactly affect them, would they? They'd just be unable to use it either way, but it would benefit those with a DS/DS Lite.