Acrobat 8 Pro and 9 Pro enable you to "Reader Enable" a fillable form so that users of Adobe Reader can save the form locally after they've filled it in. However, Adobe also sells the Adobe LiveCycle Reader Extensions ES2 server software that Reader Enables forms. So, why would you need to purchase the server software if you have Acrobat 8 or 9 Pro?
The answer lies in the EULA (End User License Agreement). Here's an excerpt from the Acrobat 9 Pro EULA:
So, the way I understand it, is that you can either a) send a form to an unlimited number of users, as long as less than 500 are returned to you, or b) send a form to an audience of no more than 500 people, who can return an unlimited number of instances of the form. Beyond this, you need to purchase the LiveCycle Reader Extensions product.
I was asked about the scenario of sending a form to an unlimited number of users, who would only return the form in hard copy format, not electronically. I asked an Adobe Business Development Manager about this. His reply was "If the form is returned in any format -- even print -- then that triggers the 500 response limit."
There's a lot of confusion around this issue. My hope is that this clears things up a bit.