Another screenwriter here. I am actually loving the iPad as a supplemental writing device. Will it replace my MBP and Scrivener? Not 100%, of course, but I actually have found I really enjoy writing on it... for first drafts.
The most important part for me is just getting it down, and strangely the iPad (with its full-screen apps and not-quite-true-multi-tasking) helps me focus for longer periods of time. I sync Scrivener to Dropbox and then use iA Writer to draft up each scene. Is it formatted properly? No. However, using John August's Fountain syntax, I can easily convert the text files into .fdx or .pdf. Plus by using straight text files, I can work on the script anywhere on any device.
I also sync Scrivener with Index Cards, and it provides a really fantastic outline method. Its a virtual cork board, and it means I can rearrange scenes at any point in time.
Also, many of you guys are forgetting one of the most important aspects of being a writer, and thats being a reader. The iPad is far and away the best device to read .pdf screenplays on.
If you only have a single device, of course a MBP or MBA will be the best choice. However, together, they can really make the ultimate pair.
First drafts, outlines, and reading - iPad (with BT keyboard)
Revisions, and formatting - MBP/MBA