Background notifications too?

I'm surprised - I wrote a Twitter client in 2009, and I couldn't find a way to make that work. I didn't think J2ME could do that at all, to be honest.

Maybe some form of this... ?

http://www.javaworld.com/article/2071753/mobile-java/push-me...

Yep, that'd probably be it. You could listen out for SMS (on particular port), TCP and UDP connections if I remember correctly.

Yep, SMS push. I did some prototyping using this tech for a telco company here in Sydney back in, like, 2005 (9 years ago!). It actually worked quite well.

Even I dont know exactly how it works. I think Nokia gave them access to some private API.

J2ME is not Nokia specific.

No, but it is not uncommon for J2ME apps to have a different package for certain vendors, or even for specific series or models of certain vendors. Those packages often utilize vendor-specific APIs or work around limitations or quirks in their J2ME stack. Opera's J2ME application took this to extremes, with dozens of different versions of a specific release.

I know, once upon a time (2003), I did some J2ME development for Sharp and Nokia handsets.

Is the J2ME version of WhatsApp only available on Nokia devices?

Same on Symbian. WhatsApp is easily one of the top 10 best applications available on Symbian. They are still pushing out updates too.