I know Costco offer lifetime warranty for TVs as long as you remain a member; which is a pretty good deal! I wonder if the same applies to their computers? I would assume so, if so - it would be another reason to purchase from them. I might try to confirm this myself.
Prices are cheaper there e.g. $2040 for the rMBP 8GB/256Gb (non TB) model (cf. Apple - $2129); but with JB 10% off etc sales you can get better.
But: they did have the base 15 inch rMPB with TB (16GB/256GB) there for $3100 (another few hundred off their already discounted price) for a couple of weeks until end of Feb... (cf Apple $3599). Normal Costco price for that model is $3399.
Not sure what iMac prices are like but I will have a look and post up here next I am there. Hopefully they have one of those "another $200-$300 on top" off those type deals at some point.
Fill up petrol while you are there purchasing that single item; and you have already clawed back another $10 easy... I get 20-30c off for PULP compared to regular service station prices in Canberra. 50L fill will net you $10 back already.