(detrimental reliance) Res. 2nd Contracts sec 87(2) - "an offer which the offeror should reasonably expect to induce action or forbearance on the part of the offeree before acceptance, and which does induce such action or forbearance is binding to extent necessary to prevent justice." Basically, if the offeree has detrimentally relied on the offer and that reliance was reasonably forseeable by the offeror, then the offer is not revocable. This situation mainly comes up in the contracting business when a general contractor makes a bid to an owner while relying on the bids of sub-contractors.
Promissory estoppel has been taught by my professor as a substitute for consideration... and it makes sense that way. If there is assent, offer, and acceptance, the contract will fail for lack of consideration because one person has suffered a legal detrimental by their promise, while the other person has suffered no legal detriment. That is where PE kicks in, if the offeree relied on the promise to their reasonable and forseeable detriment then the agreement can be enforced. However, if you look at PE as a substitute for a contract in general I do not think it will change the outcome of the analysis either way...it is apples and oranges.