Open resolvers seem to be a problem as defined here:
http://dns.measurement-factory.com/surveys/openresolvers.html
I have a new client who is coming from one of the companies on this list of known open resolvers:
http://dns.measurement-factory.com/surveys/openresolvers/ASN-reports/20060923.html
We shall see if and what problems come out of all of this.
Here is some more technical information about open resolvers:
http://condor.depaul.edu/~jkristof/slides/dns-ctinetseminar.pdf