Essential oils including lavender can be toxic to pets especially cats due to their livers working differently to ours... even inhalation, as would happen if you were putting it on carpets etc can be risky it used to be considered ok but now there are lots of people advising extreme caution
one quite sensibe webite can be accessed as follows
http://cats.about.com...ng/a/aromatherapy.htm
I do not know of any really good research on aromatherapy oils ad their benefits/dangers as far as I am aware there have been scientific trials only fairly weak poorly designed studies mostly quite subjective qualitative and done by aromatherapists so unlikely to be without bias. These have either been in humans or test tube studies where lavender for example seemed to be cytotoxic ie it killed cells,
I love aromatherapy as it seems to help with a number of things but cats are physiologically very different to us and what is good for you might be bad for Tiddles