The lawyers amongst us are leaping for joy. I happen to be a law convert. So ill try and explain why we're happy!
Promissory estoppel is a legal defence (a so called shield). When a party (A) intending legal relations promises not to assert their strict legal rights, and another party (B) moves to rely upon this promise, that party (A) is estopped from enforcing their rights (against B) by way way of promissory estoppel.
It goes something like this: Now MS has promised not to enforce their C#-rights , and people rely on this promise, such as start development/deploying C# applications because of this promise, if the case came to court, MS's argument would be estopped by a defence of promissory estoppel.
It's a little more complicated. For instance it must be inequitable for B if A reneges on their promise (fairly clear if they suffer a disadvantage or loss as 'one who comes into equity must come with clean hands'), the promise must be clear and unequivocal (I'd say yes), there must be a change in reliance on the promise (yes), and it is a shield not a cause of action (in other words, we can't sue MS for revoking the promise, we can simply aovid being sued).
However, things get a little confusing. MS have declared that this promise is unilateral, in other words, it is a promise to the world without the need for a formal agreement. Such things are valid in the eyes of the law, and enforced by the fact promissory estoppel acts as an equitable remedy - there is no need for consideration, a key ingreediant to the traditional offer/acceptance/consideration contractual model.
Promissory estoppel is a common law principle. It's basis in England is from Lord Denning's High Court decision in High Trees.
Law bit:
In High Trees, due to WW2, the claimant ("High Trees") agreed to reduce rent for a block of flats. After the war, the claimant brought action seeking the past and future rent. Lord Denning said "When a promise is made that is intended to be acted upon, and is acted upon, you are estopped from going back on it."
In High Trees Denning referred, not to a previous case of Foakes v Beer (about the part payment of debt), but Hughes v Metropolitan Railway to establish his basis for promissory estoppel. In Hughes, it was held that the opening of negotiations for sale of a property had an implied promise not to enforce an outstanding notice of repair that would forfeit the respondents lease.
Key to the criticism over Denning's decision is that Hughes only suspended rights, whereas High Trees may extinguish them. This position has recently been approved in the UK by the House of Lords in Tool Metal Manufacturing Co. Ltd - the promisor may revive rights by formal notice, unless it is impossible for the promisee to resume his original position.
Is it impossible to resume the original position prior to this agreement? We're talking about computers here. The agreement has come now, not several years ago. Consider Mono as it is now, as the original position. This is such a contentious area when you consider MS can revoke the promise, creating ambiguity, and because under Coombes v Coombes promissory estoppel is not a cause of action, the Mono community cannot sue MS to enforce this promise!
Matt