We can safely assume that IPv6 only nodes will be able to access IPv4
only services, e.g. by using a NAT-PT server provided by the IPv6
I assume that IPv6 ISPs will provide various translation services
to allow their customers to access IPv4-only services; however, I
hope that NAT-PT is not imposed "by default". This would greatly
increase the problems that are already caused by having multiple
views of DNS at different sites.
The converse is not true: there is no way an IPv4 only
node can understand an AAAA record and associate it with an IPv4
address. If the IPv6 node wants to provide services to the IPv4 only
Internet, it must somehow obtain an IPv4 address and document it in the
DNS. Depending of the service, we have three possibilities:
1) Basic services, e.g. HTTP & Telnet: document an IPv4 address in an A
record for the server;
2) SMTP: document at least one IPv4 capable server in the MX list;
3) Services accessed through SRV records: document at least one IPv4
capable server & corresponding port in the SRV list.
And SRV only works for applications that were designed to use it.