I was looking few days for something like this and look what I found!
with this app you can follow your friend online activity and get a notification to your iPhone when the friends you are following are online , and also when they checked in near you!!
Exactly what i was looking for!!!
You should try it , i love it..
Who's online :
Who's Online for iPhone 3GS, iPhone 4, iPhone 4S, iPod touch (3rd generation), iPod touch (4th generation) and iPad on the iTunes App Store
There is a free version called Who's Online Lite