intherealworld,
I think we're going to have to agree to disagree on this one. I don't think it's ok for me to cross a picket line - however, I can't speak for others, and if they do then that's something between them and their conscience.
The "but if it's on my watch you certainly keep your head down! " comment is just unnecessary. Just because you can't think of many reasons to cross the line (neither can I), it doesn't mean there aren't any.
It is absolutely right that we all feel free to voice our opinions on this matter - this is probably the most important issue that any of us will face in our professional careers.
I just hope this discussion is hypothetical and it doesn't come to strike action at all. I don't wish for anyone to be faced with that dilemma.