We were able to trade in my DS' DS for the DSi for $75, I think, at Gamestop. Not sure if they are still doing that. I'm not sure how that would work if it is supposed to come from Santa, but it's something to think about. My DS enjoys the camera part of the DSi.
So far, DS6 has only asked for a Saints uniform (He wears his LSU one ALL the time and acts out the football games as we watch!), Star Wars Legos (Specifically something with Anakin), and candy. He already has so many toys, we're actually thinking about getting him a fish tank for his room.
I think that was a special. I asked the other day and they were only offering $30 in trade for the DS. We want to keep ours, though, because his little sister always wants to play it.
Regarding the DS vs DSi:
The DSi has the camera (though it is supposed to be hard to get the pictures off) and the guy at Gamestop said they can insert their faces into games, decorate them, etc.
They can also access the internet with it. I asked about parental controls and he said it does have some.
I think it also holds music??
My son's DS has been well taken care of but it's having some problems so we're going to go ahead with the DSi for Christmas. He doesn't play with toys at all anymore. He's into sports and electronics.
To answer the OP, my son will be almost 9 at Christmas. I'm having a hard time figuring out what to get him. He really hasn't asked for much.
DSi
Pokemon games for the DSi and Wii (he has all of them that are currently out)
Pokemon cards
That's all he's asked for. Other things he's mentioned in the past or that we are considering:
digital camera
ihome thing with the radio built in for his ipod Nano
new Saint's jersey
private or small group baseball lessons
My daughter is 3 and I have NO clue what I'm getting her. She plays with toys a little, but there isn't anything she really loves enough to buy more of. She does love to paint so she may get an easel and painting supplies.
The toy that gets the most play here is a Fisher Price dollhouse, so I'm thinking of getting the Fisher Price swan castle.
She'll probably also get some books.
We're also considering a bike.
) know not to even bother asking!!!! 
every year i tell them that "this year" will be a smaller christmas, but we always go over board. i'm afraid this year will be the real thing! 
i'm just thankful they aren't little and into santa anymore! actually, i take that back. it would be much easier/cheaper to just get some toys!!!!!!
i love christmas!
