I think many people might not know this, so I post this tip.
I have never used Hacktivate since 3G came out.
I only tested on At&t iphones, but I am sure others should work too.
For iPhone 3GS or iPhone 3G, if you have a nonworking AT&T sim somewhere.
Just jailbreak without hacktivate, then slide in AT&T sim and connect to iTunes to go to homescreen.
Phone will not be activated yet.
Install ultrasn0w, and put in your carrier sim.
When you have signal, phone will be activated. A message will pop up and tell you phone is activated.
This way, you will have official activation with youtube and push working.

Remember that you don't need an official active sim card; a nonactive official sim works just fine to by pass the activation screen.

Wow if this really works for me too, it's awesome !!! So, now I can use an unactivated custom firmware (using the O2 sim that I don't use ) and then unlock, put my own sim and have push notifications That would be great, I'm going to try this !!! Has anyone else tried this, anyone to confirm that it works ???

Edit:

I've tried the method but unfortunately it didn't work for me . As soon as I insert my official sim, to be able to connect to iTunes (i.e. to go to homescreen), my iPhone is instantly activated, which is not desired for this method. After that when I unlock and insert my sim, iTunes says that the sim is not supported.

I guess the reason why it didn't work for me is that the official sim I used is still active (I bought my iPhone about a month ago, although I don't use the official sim, it still works). Because in your method, the official sim used was nonactive and was only used to bypass the activation screen (iPhone was activated after you insert your own sim). I think this is why it didn't work for me.

Anyway, I installed the activated firmware again and everyhting's fine, I guess I should forget about push notifications at the moment Maybe until my Pay and Go sim becomes nonactive

Has anyone else tried this method ??? Please let us know if it works for you or not ???