I have been invited to the final interview round of a boutique consulting firm. I will be having interviews with Partners and "informal meetings" with consultants. Are these informal meetings nothing but interviews by stealth? Or is this is a sign that my offer is as good as certain - why else would they make me meet other consultants "informally"?

Yes, they are interviews. They are described as "informal" as they are personality-fit interviews.

If they like you, you've got a job. If not, you haven't. Just be yourself, as anon said they're about personality fit. If you don't fit, then you wouldn't want to be there.

I agree with Anon, it is a part of interview and I have been dropped during the last interview which was with Partner and VP. So just be yourself and try to relax. They might ask you very simple questions but they would like to confirm your fit within the organisation.