The answer is that I misunderstood the instructions (or lack thereof) on the web form where I register the feature licenses I purchased.
The form asks for a PID and serial number. Although the instructions on the form do not specify this, both of these pertain to the hardware appliance, not the license; i.e. I entered the PID of the license--which the form accepted--but the resultant key file could not be installed on the appliance.
I opened a TAC support case and had the license re-keyed with the PID and S/N of the appliance. Now it works.
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
[toc:faq]Introduction:This document describes details on how NAT-T
works.Background:ESP encrypts all critical information, encapsulating
the entire inner TCP/UDP datagram within an ESP header. ESP is an IP
protocol in the same sense that TCP and UDP are I...