Before Chanukah I touched on the topic of toch k'dei dibur and want to get back to it because of a few good questions in the comments (thanks Micha). There is a basic dispute between Rabeinu Tam and the Ran as to the source of toch k'dei dibur. Ran (Nedarim 87) holds that tk"d is a din d'oraysa based on the all too common phenomenon of a person's brain taking a few seconds to catch up to his mouth. I person who screams "buy" to his broker may turn around a second later and scream "I meant sell!" Since it is mental agreement and coming to terms and not just speech alone which seals a transaction, the few seconds until a person's mind (and not just his mouth) is settled allow for retraction or clarification.
Rabeinu Tam disagrees and holds that tk"d is a takanah derabbanan. Really, we should all be bound by our words and have no opportunity to retract or clarify. However, Chazal recognized that in normal business transactions there are times when we deliberately pause mid-speech, e.g. a person sees his Rebbe and wants to say "Shalom Alecha Rebbe." Therefore, Chazal allowed the time it takes to say those few words as a natural break after which the person can resume his previous speech and retract or clarify as necessary.
The gemara notes that there are certain exceptions to the rule of tk"d, e.g. kidushin, gitin, avodah zarah, megadef. According to the Ran, the reason for these exceptions is simple. The rule of tk"d is based on the assumption that a person sometimes speaks before thinking. These exceptional cases have such serious consequences that a person undoubtedly is fully aware of what he is saying and doing while speaking. It is unimaginable, for example, that a person should utter "harei at mekudeshet li" to betroth a woman without fully being aware of the impact and consequences of those words.
According to Rabeinu Tam, there seems to be no clear reason why Chazal should have excluded any cases from the takanah of tk"d. The Rashbam (Baba Basra 130) writes that these exceptions are simply a "chumra derabbanan", i.e. because of the severity of the prohibitions involved and the issues at stake, Chazal decided to stick to the letter of the law and not make any allowances for tk"d retraction or clarification.
So much for the basics. The next step is to collect and review the cases of tk"d discussed in various sugyos and see which model -- the Ran's or R"T's -- fits better. More to come, bl"n.