I recently created a project with Sparkfun's Wifly shield that sends push notifications to my iPhone when certain conditions are detected. I decided to go with this approach since I had the infrastructure in place and the code to make this work on the Arduino is dead simple. I created a blog post that details what I did here http://ronguest.com/blog/2011/05/arduino-push-notifications-my-solution/.
It involves running a PHP script on a server and the use of the Prowl app on my iPhone. The details of the post are specific to my setup but the components and outline are similar even if you are using different platforms. I thought it might be useful as I see folks regularly post about the use of email and SMS for notifications and this is a viable alternative in some cases. From an Arduino standpoint it doesn't really matter what Ethernet or WiFi shield you use.