I guess there are few possible answers as to why it won't be broadcast.
1. Money. A live webcast is not cheap, that's for sure but it wasn't any cheaper in the past. So, money could be an issue, but I'm sure it's the only reason.
2. Bandwidth. As already been said, because of recent "rumours" this year's keynote might attract more attention than usual, so Apple didn't want to annoy the audience in front of their computers with a bad quality broadcast. Sure, they could extend their bandwidth but that would increase costs.
3. "Embargo". Apple, for some reason doesn't want to spread the word of new products through the broadcast. They might want the press to get their official press release first. A reason for this could be they want the press to get it right, i.e. Apple want them to inform their readers/viewers in a way that seems appropriate to Apple. An example for this could be some journalist who doesn't know Macs might see specs of the new headless Mac and tell his/her readers how bad the specs compare to contemporary PCs.
4. Fear of mishap. This has also been brought up already. They don't want to risk an embarrassing situation if something goes wrong. Tiger demo? iWork demo? Any other product?
5. Basically same as 4. In case those rumours were just rumours and therefore not true, Steve Jobs might give some sort of explanation (plus some *friendly* comments about rumour sites) to the audience and even more importantly to the press (through a press release). So they might cut that part later.
So in a nutshell, the reason why they are not going to broadcast the keynote life could just be a combination of 1. and 2. But I have a feeling, that at least one of the other points could be part of the reason, too.