The answer as of now is No.
This file is encrypted and can only be opened with Acrobat Reader.
richard@ubuntu:~/Desktop$ pdfinfo n-400.pdf
Title: Appication for Naturalization
Author: USCIS
Creator: Adobe LiveCycle Designer ES 8.2
Producer: Adobe LiveCycle Designer ES 8.2
CreationDate: Thu Nov 15 16:21:11 2007
ModDate: Tue Jun 21 11:36:34 2011
Tagged: yes
Pages: 1
Encrypted: yes (print:yes copy:no change:no addNotes:no)
Page size: 612 x 792 pts (letter)
File size: 110428 bytes
Optimized: no
PDF version: 1.7
The only unencrypted element is the following text, found using pdftotext
richard@ubuntu:~/Desktop$ pdftotext n-400.pdf l.txt && cat l.txt
To view the full contents of this document, you need a later version of the PDF viewer. You can upgrade
to the latest version of Adobe Reader from www.adobe.com/products/acrobat/readstep2.html
For further support, go to www.adobe.com/support/products/acrreader.html
There are lots of people affected by this.
My solution was to install acroread, fill in and print the form (the option to save wasn't available either for the form I was using), and then remove acroread.
www.ussaram.com/N-400.pdf
which seems to provide a valid pdf with fillable forms (I don't know if it's valid/updated though). – Pavlos G. Aug 20 '11 at 13:59