Actually it will and it's on the way. It's been mentioned more and more lately. I haven't seen an estimated date or anything but I keep seeing 3rd party companies mention getting their demos up on the Shop Channel and stuff. So do hold your breath
Even if they hadn't mentioned it, it's not a huge stretch to think it'd happen considering Nintendo's been allowing the same kinda thing with the DS and the Download Stations for a while now. I'm just figuring of all the things I want from Nintendo, I'm happier to think they're focusing more on Metroid Prime 3 and SSBB at the moment.
I'm gonna go out on a limb and make a total guess but I could see Nintendo launching a Wii Demo Channel with Super Mario Galaxy as the first download. Probably just a gameplay video since Mario games are usually not the types to have demos so much.