Hey, look - I'm not even waiting for the official release of Kaladesh! :)
Now that
the new IPG has been released, and
Toby's talked about the changes, we can address the original question - by applying the new wording in the IPG.
The reminder text on cards with Fabricate may cause us some heartache, as it doesn't quite match the rules text (roughly, “put +1/+1 counter(s) OR make servo tokens” vs. “put +1/+1 counter(s); if you don't, make servo tokens”). However, it is definitely a trigger that specifies a default action.
And, from the IPG: “If the triggered ability specifies a default action … give the opponent the choice to resolve it choosing the default option.” I know, that sentence may need a bit of explanation - maybe next time? - but the summary of changes in the Appendix confirms what I'm about to tell you: “Default-option triggers let the opponent choose whether to resolve it.”
So, I play a creature with Fabricate 2, and get distracted (probably by you, you motor-mouth gamesmanship-type person, you!), forgetting to choose an option. On your turn, you attack into what appears to be a clear board, then I remember the 2 servo tokens I was counting on as chump blockers. You have a choice: you can let me resolve the default option, or not. I'm guessing you're going to reward my forgetfulness by choosing “not”, so I don't get my chump blockers. (See how you are??!?!!!? grrr…)
d:^D