You have two options for "offline" PDF workflow with the iPad.
One is to use an Acrobat Email workflow, not collecting responses in FormsCentral but adding a Submit button in Acrobat to email the form responses to you. When users fill out the PDF and click Submit it will queue the email, when online again those emails would be sent and you'd get the responses.
Using FormsCentral is a little more work in the offline case, see this response from Genevieve where she describes how to do it: http://forums.adobe.com/message/5339644#5339644
That info aside, the Submit button in a FormsCentral PDF should work fine in Adobe Reader Mobile for iOS. Are you sure users are opening the PDF in Adobe Reader mobile? The Submit button would be missing if Preview was used to view the PDF on the iPad.
Thanks, Josh. So I think I figured out what I was doing wrong and maybe this specific detail will help other users- for the email attachment, tap the pdf form attachment BUT HOLD IT for a couple of seconds until the "Open in" pop-up window appears. Then select "Open in Adobe Reader."
I was opening the pdf attachment and then selecting the "Open in Adobe Reader" which gave me active form fillers but no submit button, and hence my confusion.
It should not matter at all how the PDF gets into Adobe Reader, unless it is being opened and modified/saved by Preview before being opened in Reader. Assuming the PDF hadn't been modified, opening straight from e-mail and clicking "Open in Adobe Reader" should work just fine, I just double checked it works to ensure there isn't a new issue and it works for me.