I'm catching up on old posts (my epic trip did not include internet) and buried in the comments of this post by FSP is a discussion of conference attendance by former students.
I went to two separate conferences in the year after I completed my thesis. I technically graduated after attending those conferences, but that was a formality - I graduated nine months after I successfully defended my thesis, after all.
I never considered it strange or unethical that I attended conference as an ex-student. For one conference, I was paid out of my advisor's slush fund (er, big industry grant) to present some interesting results from my thesis. Who would present it other than the person who did the work? For the second conference, one of the entities that paid into the grant wanted to showcase the work they'd been supporting, and they paid the travel costs for a representative of the research group (me - I happened to be available, since I didn't have a job at the time we applied to the conference) to give an overview of some of the cool stuff we'd done.
Is this unusual? I think it was fairly clear that I was an ex-student and that the work was representative of the institution I'd just left, so I didn't see any problem with it. But maybe I'm in the minority...