I am heading to Seattle for a conference - entering the country using the Visa Waiver Program. I would like to make a quick trip - probably one day or two at the most - to Vancouver and then come back to Seattle before flying home. Can someone tell me if I may expect to have problems coming back into the USA with my waived visa?
If you are able to visit the US on a visa waiver then you are probably able to visit Canada without a visa. You should probably check to be on the safe side. Likewise there should be no problem with exiting and re-entering the US on a visa waiver.
You will have no problems at all.
Officially your 90 day WVP entry into the US will not be reset by the quick visit to Canada, although realistically I'm not sure how this is enforced nowadays. (It used to be that you would keep the I-94W form that you received when you first entered the US, and re-use it to re-enter from Canada, but as I-94W's no longer exist this may not happen).
Regardless of whether they treat it as two entries or one, it's not a problem.
You will also need to confirm that you do not need a Visa to enter Canada. Presuming you're an Australian, you should not need one (presuming no crime record/etc, but if you can get into the US under WVP you should have no issues getting into Canada either).