which they obviously weren't. I played around with this for a while, also trying my yahoo user id and getting nowhere.
Eventually I tried another flickr plugin on another site and got identical behaviour - at which point I logged a support ticket with Flickr to ask what I was doing wrong. It turns out that "username" in this case means this label here:
Never noticed that before, and its certainly not how I refer to myself, so I'm a bit confused - but hey my plugin is working! I hope this helps someone (probably me, next time I try to plug flickr in to something else) confused by flickr seeming to think you have the wrong username!