This might be helpful http://www.usembassy.org.uk/cons_web...l.htm#nineteen

In theory, you can leave and return again within a day and have as many visa waiver entries as you wish. In practice, you're likely to have a lot of questions from the INS who will probably ask why you haven't applied for a B2 visitors' visa if you're entering the US too many times on the visa waiver.

As you say, the immigration official on the day has discretion over your entry. Only the INS can answer your last question but it's their country and they can make whatever rules for entry that they see fit, in the same way that we do in the UK.