I'm a film director and writer - my screenplays have won a few awards and I'm working on my first novel now - my opinions -
First off, I agree with everyone above that a MacBook Pro is a writer's best friend, however, be sure to buy some COOL FEET so you don't get carpel-tunnel -
http://www.bluelounge.com/products/coolfeet/
Second, an iPad is great for organizing ideas and putting thoughts together - IT IS NOT GOOD FOR WRITING them out --- the best application for putting ideas and plots together is CORKULOUS for iPad -
http://www.appigo.com/corkulous
Third, I agree with everyone who loves SCRIVENER - that application (on MacBook, not iPad) is EXCELLENT - I feel sorry for anyone who writes a novel using Word - DO NOT DO THAT - Scrivener makes the organization and idea creation so much easier and it's all integrated
http://www.literatureandlatte.com/scrivener.php
Lastly, ADOBE just released an app for the iPad called IDEAS which is great for drawing out notes --- PENULTIMATE is also good for notes on the iPad, although I think PAPERDESK is better than PenUltimate
Enjoy all! Good luck on everyone's writing!