For another angle on the risks of browser prefetching, read this paper by Srinivas Krishnan and Fabian Monrose. The authors describe algorithms that allow a hacker with access to a shared name server's cache to determine -- with remarkable accuracy -- what terms users with prefetching browsers are searching for.
This is also a good reminder of the importance of locking down queries to your name server. Through judicious use of BIND's allow-query and allow-query-cache sub-statements, you can prevent unauthorized queriers from snooping your cache, whether or not your users use prefetching browsers. Recipe 7.12.2 in my DNS & BIND Cookbook shows how to do this with just allow-query. allow-query-cache actually makes it a little easier, but that exercise is left to the reader.