There has been lots of debate as mentioned. The line was apparently improved and kept in by FFC because it fit the plot. Here's what we know:

1) Michael didn't order the hit.
2) Roth organized the hit, and admitted to it in a roundabout way in the rant about Moe Green.
3)The more you think about this point, the more irrelevent it becomes to the plot. This is not really of much matter since if it weas a legit hit that was botched, Roth totally made up for it admirably. If it was a set up, it worked out wonderfully. I would like to venture that it was a straight hit because Roth had already arranged a hit on Mike in Cuba, so he would most likely want to take out Pentangeli to consolidate more power for the Rosattos. It has been suggested if it was indeed a straight hit on Pentangeli that Rosatto said "MC sends his regards" or whatever he says to shake him up before he dies; that would be a pretty harsh thing to say before you were capped.