1. The site claims to be operating under the following official program: OMB # 1405-0177
Your participation, while voluntary, will be essential to the success of the study. This interview is estimated to take less than 10 minutes to complete. Your answers will be kept private to the extent permitted by law and identifying information will not be released outside of the Department of State. Additional information on the study can be found in the Federal Register under OMB control # 1405-0177, expiration date 12-31-2016, at https://www.federalregister.gov/. The estimated burden is 10 minutes. If you have comments regarding the interview length or content, please email PassportStudy@state.gov.
2. Knowing whether the site and your letter are actually part of that program, or merely are a phishing attempt under the guise of legitimate program, is a question for Information Security SE, not Skeptics).
One possible indication that it's legit is that its Domain Name was registered back in 2012, 1 year after OMB proposal was registsred with the government:
Domain Name: PASSPORTPLANNING.COM
Registrar: GODADDY.COM, LLC
Sponsoring Registrar IANA ID: 146
Whois Server: whois.godaddy.com
Name Server: NS09.DOMAINCONTROL.COM
Updated Date: 07-may-2013
Creation Date: 27-jun-2012
Expiration Date: 27-jun-2017
3. The program itself is legitimate, and is registered with OMB.
30-Day Notice of Proposed Information Collection: Passport Demand Forecasting Study Phase III, OMB Number 1405-0177
A Notice by the State Department on 08/29/2011
Notice of request for public comment and submission to OMB of proposed collection of information.
The Department of State has submitted the following information collection request to the Office of Management and Budget (OMB) for approval in accordance with the Paperwork Reduction Act of 1995.
Title of Information Collection: Passport Demand Forecasting Study Phase III.
OMB Control Number: OMB Number 1405-0177.
Type of Request: Revision of a Currently Approved Collection.
Originating Office: Bureau of Consular Affairs, Office of Passport Services CA/PPT.
Form Number: SV2011-0010.
Start Printed Page 53705
Respondents: A national representative sample of all U.S. Citizens and U.S. Nationals, who are 16 years of age or older, that are eligible and entitled to a United States Passport product.
Estimated Number of Respondents: 4,000 survey respondents per month.
Estimated Number of Responses: 48,000 survey responses annually.
Average Hours per Response: 10 minutes per survey.
Total Estimated Burden: 8,000 hours annually.
Obligation to Respond: Voluntary.
Please note that this is a research by a private entity (therefore,
.com in the address is OK) on behalf and under grant from US government.
However, that doesn't mean that this specific website and this specific letter are indeed from the people whose proposal OMB approved and not some phisher impersonating them. This can only be answered on Infosec.SE.