I've been using
dyndns.org. For the moment, not for a domain name,
just using a hostname under
dyndns.org. (Nice -- it's free.) But
they offer domain names registration too, I believe, along with the
dynamic address support so you can have a name even though you don't
have a static address.
I believe they mentioned supporting .aero as a TLD -- I've been
tempted by the hack value of that one, though they are rather pricy.
paul