Correction, Obama's page states you might not get a personal response to "Mail" below the mailing address. However, when you go to the web questions link you get sent to a page requesting your personal information with no such disclosure below.

You can contact us by mail at:
Obama for America
P.O. Box 8102
Chicago, IL 60680

Due to the high volume of mail Obama For America receives, we cannot guarantee the return of items sent to the campaign. In addition, we regret that Senator Obama cannot sign items requesting his autograph due to demands on his time.

http://my.barackobama.com/page/s/contact2

Here is McCain's contact page which makes the following disclosure for all contacts.

http://www.johnmccain.com/Contact/

Thank you for your interest in John McCain 2008. Please fill out the form below to send us your questions and comments.

Due to the high volume of comments sent to John McCain 2008, we may not be able to offer you a personal response to your comments.