Apple appears to be in the early -- yet strangely public -- stages of testing Notification Center-style banner alerts on iCloud.com, the web portal they provide for remote email, calendar, and contact access.
Apple introduced both Notification Center (as part of iOS 5) and iCloud at WWDC 2011 and have since added Notification Center to the OS X 10.8 Mountain Lion developer preview. Given Apple's recent push towards providing a more consistent user experience across their platforms, it makes sense for the web to be included in the mix.
However, being only a month away from WWDC 2012, do iOS 5-style banners for iCloud.com mean there's less of a chance we'll see better banners in iOS 6 this June? While less obtrusive than the pre-iOS 5 modal alerts, iOS 5 banners still aren't as functional or elegant as webOS' were years ago.
Sometimes a web test is just a web test, so I won't read anything more into this for now. Parity between OS X and iOS and the web will be great.