I found this, which seems relevant:
http://www.law.cornell.edu/uscode/text/47/617
Apparently this requirement is somewhat old. Also, notice that it includes software, which I find interesting.
The relevant exemption, which appears to apply to ereaders is:
(B) is designed for multiple purposes, but is designed primarily for purposes other than using advanced communications services.
The primary purpose of ereaders does not seem to fall under the definition of ACS, so I suspect that this will be an open and shut decision and an exemption will be granted.