The upstream kernel got source port UDP randomization (where no port is specified) in 2.6.24.
You can see this in practice by testing distributions like Fedora 8 or 9 where the glibc stub
resolver will use a different source port on each request, therefore mitigating this issue.
Users of older kernels will either need a backported patch to add this functionality, or
changes to glibc if they want UDP source port randomization.