I think that if apple was going to revise their notification system the most important thing would be to make it not interfere with the current application. My guess is that we will get a revised notification system with OS 5.0 and I think that it will probably reside on the top because applications are already designed to be able to run while the god bar is doubled (Like when your in a phone call or recording something). So my guess would be the god bar would swing down to its phonecall height and the notification would swoop in, swoop out, and the bar would go back up.
Here is my reasoning for why we will get revised notifications in OS5:
2007: iPhone announced, everyone is amazed, they call it the jesus phone, but as the hype dies people start complaining about the fact that it doesn't have 3G or GPS
2008: iPhone 3G: Yay it's just what everyone wanted, it finally has 3G and GPS, oh and look! Now we have apps... But why doesn't it have copy and paste? or MMS, or a Video camera? every other phone on the market does why is apple not listening to their customers...
2009: OMG copy and paste! Look and now we have MMS and tethering too (For non US), and there is that video camera it needed to be a "modern" phone, plus now we have push notifications that will be just as good as multitasking... Or not, how could Apple not have realized we needed to listen to our streaming radio while surfing the web, who cares about how it will effect battery-life I want my true multitasking!
2010: Multitasking! and Folders! and more awesomeness, but look at this, how could apple have not revamped the notification system, its obvious that its annoying, how could they not see their customers need a streamlined notification system, what are they thinking...