I may be dim but I can't see how the user of the NaNoWriMo website retains his or her copyright in their work. This is a quote from the website:-
'You give the party (the “Account Holder”) who owns the domain name as hosted on the Service in which you post your Content (the “Hosted Area”) a non-exclusive, royalty-free, worldwide, perpetual, irrevocable , fully-sublicensable license to publish and use your Content. In addition to the right to publish, you also grant the Account Holder under said license the following rights, without limitation: (i) the right to reproduce or copy or create derivative works thereof; (ii) the right to transfer, deliver, and sell the Content, which includes the distribution via computer and networks; (iii) the right to edit, modify, adapt, arrange, improve, correct, develop, translate, in all or in part; (iv) the right to update/upgrade by adding or removing; (v) the right to film, perform or post the Content in any media, and (vi) the right to use or incorporate all or any part of the Content in any products or services of the Account Holder (whether or not associated with the Service). Except as described in our Privacy Policy, neither the Company nor any Account Holder will be required to treat any Content as confidential. By posting Content you hereby waive any and all rights to be compensated by UserVoice or any Account Holder for such Content.
In order to operate and improve the Service, the Company needs the right to make certain uses of your Content as well. Therefore, when you post Content in any Hosted Area, you also agree to grant the Company an irrevocable, perpetual, worldwide, royalty-free, fully sublicenseable, non-exclusive license to copy, distribute, sell, publicly display, publicly perform and make derivative works of your Content on the Service and on services affiliated with the Service regardless of the form of media used or of whether such services now exist or are developed in the future.'
Apologies for the length of the quote, but how can it mean that we the writers keep our copyright in our work?
Seeing as you need to copy and paste your work to make each word count, the best thing you can do is use http://www.lipsum.com/ so nothing you've written is ever posted on the site. It works by typing how many words you've already typed, then it will generate a text with the same number of words. You can use this to enter your word count. This is mainly for people who are writing on paper, but I see no reason why anyone who is worried about any part of their work being 'stolen' can't use it.
I hope this helps.
A cynic would say it's a good way to borrow/steal would-be authors plot and storylines.
Many thanks for all the answers. I've left it to very late to make a decision to join in - November 1. So that's my problem. But, confusingly, so many people on the Writers and Artists' website are all for doing it or have done it, and pay tribute to the idea.