Screenwriter here. I'll probably get an iPad3 for reading scripts and eBooks, but when it comes to actually writing, nothing beats the MacBook Air.
Screenwriting is so keyboard-centric that I can't imagine doing it on the iPad. I've found the touch-screen isn't responsive enough, and if you need to use some sort of integrated keyboard / case thing, that kind of defeats the purpose. Plus, in my workflow, I like to have MacJournal open with my outline, and Final Draft open to write in. The lack of ability to run two programs at once hurts the iPad.
I'm using Goodreader for PDF's, as well as FDX Reader. I didn't buy the Final Draft program for iPad. I'm curious to see what their solution for writing on the iPad is, if they ever bother to come out with it. It's been two years! Final Draft stinks as a company. I use their Mac software, but it's a buggy, crappy program. I've used Movie Magic and I liked it, but nobody uses Movie Magic (or Celtix), so there's no real point in writing with those programs.
In conclusion, my opinion is that the iPad3 will be great for reading scripts, and stink for writing scripts.
Bob