I've been helping a high school student with a senior project related to an app she is developing in HTML5. Push Notifications were desired and are of course one of the more useful aspects of having a mobile solution. Unfortunately, Push Notifications (without using a third-party solution) require a native app, at least at the present time for a broad iPhone/Android audience.
PhoneGap may offer a solution, but I've started on a research project into alternatives for developers wanting to offer a nice Web App solution supported by a third-party Push Notifications solution. Here are some links that I've found so far:
There are no doubt some text messaging service options that would provide another route for a solution that requires a capability like Push Notifications. I'll append these here over time.