In case anyone else is wondering about the MDR1 gene here is some info I got from someone on an Aussie board:

MDR1 (Multi-Drug Reactivity 1) :
A mutation in a gene called MDR1 has been found to cause toxic reactions to several drugs in those dogs with at least one copy of the mutant version of the gene. Dogs with two copies may be even more sensitive to some of the drugs. This mutation is present in some Australian Shepherds.

A screening test is now available. For information on the test and a list of drugs that cause reactions: http://www.vetmed.wsu.edu/depts-VCPL/.

In Europe, the test can be obtained at http://www.vetmed.uni-giessen.de/pharmtox/index.html

To see the results of the study that demonstrated the presence of this gene mutation in Aussies: http://www.pnas.org/cgi/reprint/0402374101v1