It's just my opinion, and the lawyer genes in my blood, but I don't count on anything until there's ink on paper (and/or a plane ticket when applicable). I am hopeful and optimistic about gigs, but I won't make any arrangements that can't be adjusted or that limit my other options until there's a formal commitment. I've seen it happen to SM friends taking gigs by word of mouth, internet postings, or even gigs that are coming through a series of interviews at very reputable professional theatres. Until there's a contract, there isn't a gig.
I've "accepted" jobs over the phone, with the understanding that a contract is coming later, plenty of times - especially for unusual or last-minute gigs. But I just don't count on anything until we've got a formal document.