I've seen this phrase thrown about as a dismissive slur, and it has always bothered me a bit.
I think it is rather harsh to demand another poster be present whenever a second poster responds. Posting on the Planet is a passtime activity. I know I have often had the situation where I have submitted a post, and then realized there was something more pressing that needed to be taken care of - something on the stove, an errand to run, etc. Of course, I usually don't say anything too provocative so no one calls me on it. Still, I think the same situation applies to others.
A more realistic example of seagullism would be BigSexy, who posts a new thread and never comes back to it, or participates in any other discussion.
There is the flip side, of "pimping your thread". It seems an awfully narrow range we have to trod.
~not taking sides, just making observations.
...and STAY out of the WOOLWORTH's!!!!!!!!!