Subscriber Artificial Module (SAM) will allow you to do official activation on an iPhone without a stock SIM and without using hacktivation. It is written by sbingner.
The iPhone Dev Team states that, "Subscriber Artificial Module tricks your iPhone and iTunes into creating legitimate activation tickets even though you’re unlocked with ultrasn0w. This means you get the full benefit of push applications."
Benefits
Unlike hacktivation, SAM does not seem to put extra strain on the battery life of the device or interfere with push notifications as an individual ticket is created per device.