When I submit an entry using the SubscribeForm.php, I receive blank entries in the database. I notice that this has been an issue for other users and the tracking log states that its been fixed - however I am using the latest version of Silverstripe Core, UserForms and Newsletter and still experiencing the same issues.
UsersForms and SubscribeForm are not currently working very happily with one another as SubscribeForm is out of date with UserForms. See http://open.silverstripe.com/ticket/4510 . That says it has been fixed so try the super uber latest daily builds.