The conspiracy would have to be vast. So vast as to be impossible. The idea does not make sense when compared to the gospel message. Christianity was hostile to Roman authority at it's core. While it taught to be obedient to Roman rule, it also taught that God would overrule it. If it was a Roman invention one would expect to see a far greater love for Roman authority commanded in the gospels. Jesus message was a message of HIS Kingdom and not the Roman Kingdom. Rebel Jews would never approve of Jesus in the first place. Why would the Romans care to start a new religion when they had already beaten them on the battlefield?
There is too much that just wouldn't fit in to this hypothesis. Why would Titus be involved in this? Tacitus places the origins of Christianity and the Romans reaction to it almost a decade BEFORE Titus' victory. Pliny also wrote a letter to Trajan asking what they should do about the Christian movement. Why would Domitian persecute Christians forgetting that his own relatives had created it in the first place?
As far as I have been able to tell, Atwill is completely alone in his hypothesis. Roman history just doesn't fit in with this "conspiracy theory". It seems that he took a few points (i.e be obedient to authority, and Jesus' statements that His kingdom was not going to take over Rome at that time), and he ran with it. It seems that people are so intent on disproving the gospels, they will latch on to anything. This has conspiracy theory written all over it and seems to be similar to modern conspiracy theories.