Must you always post rubbish based on your sample of one trip??

Do you really think there are that many flyertalkers out there doing this that it would stick in the memory of a US immigration agent?

There are a relatively tiny number of people doing turnaround MRs, and in my experience (also limited, but more than yours) some explanation is always necessary if you are doing any kind of turnaround.

In doing a number of turnarounds since 9/11, I have only ever met one agent who actually understood what I was doing, as opposed to not being too worried by it. I have never been refused entry, and wouldnt expect to be, but depending on the agent may have to spend some time discussing the purpose of your trip. As always, be totally honest, only answer what you are asked, and have paperwork to support if asked. My view is, it might be unusual, but its not illegal - so there is no need to worry. Allow extra time just in case though.

I still find Canadian immigration to be much more of a hassle and really incapable of understanding (YVR told me once that I could either go to SEA or YYJ for a weekend, anything else was suspicious!)