(ignore the date in the screenshot, that's the date the app first appeared in the App Store)
Just tried it using Skype and Windows Live accounts. The push notifications didn't work in Windows Live. I'm trying Skype right now. Stay tuned...
Actually it does work. Even though when I first launched the app and I clicked "Yes" to allow push notifications, by default the "Stay Online" feature is disabled / off. Very intuitive eh? Anyway, I had to click 'Settings', then 'Stay Online', and then pick 5, 15, or 30 minutes. Bummer that it's not 24 hours. 30 minutes is way too small. I recall some other push apps limited to 30 minutes. I wonder if this is a Steve Jobs imposed limitation?
Still, it's nice to be able to receive IMs from Skype, Windows Live, Yahoo!, AIM, and more while Nimbuzz is closed and I'm running other apps (up to 30 minutes). It also sports Facebook as well. Going to add that in now and test it.
Looks like Facebook IMs work as well. Pretty sweet! My only complaint now is that I have to scroll through hundreds of Facebook friends, Skype buddies, etc.
Update 4: (9 hours later)
Looks like Nimbuzz also launched a Mac client too! Boy, they've been busy!