Summarising: Three ways of looking at it. Extracts from the Geoffrey Leech article, English Grammar in Conversation.

View 1: Spoken English has no grammar at all: it is grammatically inchoate.

(That view) ...does not need to be taken seriously, although it is surprisingly persistent in the mind of the folk grammarian. It is inherited from the age-old tradition associating grammar with the written language, and it is bolstered by examples such as the following, which, like others which follow, is from the Longman spoken corpus:

No. Do you know erm you know where the erm go over to er go over erm where the fire station is not the one that white white

View 2: Spoken English does not have a special grammar: its grammar is just the same as the grammar of written English

Conversation makes use of entities such as prepositions, modals, noun phrases and relative clauses, just as written language does. So - assuming, as many would, that differences of frequency belong to the use of the grammar, rather than to the grammatical system itself - it is quite natural to think in terms of one English grammar, whose use in conversational performance can be contrasted with its use in various kinds of writing. In other words, conversational grammar is seen to be just a rather special implementation of the common grammar of English: a discovery which does not necessarily in any way diminish the interest of studying the grammar (i.e. the grammatical use) of spoken language.

View 3: Spoken English does have a special grammar - it has its own principles, rules and categories, which are different from those of the written language.

In handling spoken language, (David) Brazil argues for a totally different approach to grammar from the approach which has become familiar through conventional focus on the written language. He argues for a linear model moving dynamically through time, and puts aside the more traditional architectural model in terms of hierarchies of units. Although Carter and McCarthy do not take this thorough-going approach, they do throw the spotlight on grammatical features of spoken language which they feel have been largely neglected by standard grammars entrenched in the 'written tradition'. They argue that structures which are inherent to speech have not been properly studied until the advent of the spoken computer corpus, and are consequently absent from canonised written grammar familiar to learners of English throughout the world: structures such as the 'dislocated topic' of This little shop ... it's lovely or the 'wagging tail' of Oh I reckon they're lovely. I really do whippets. These tend to find their raison d'être in the fact that conversation constructs itself in a dynamic fashion, giving the speaker only a small look-ahead window for planning what to say, and often inducing retrospective add-ons. Carter and McCarthy (1995) put forward a structural model for the clause in conversation, containing in addition to the core clause itself a pre-clause topic and a post-clause tail. With their refreshing emphasis on the dynamic modelling of grammar in action, Carter and McCarthy seem to be taking a line similar to Brazil's advocacy of a new grammar of speech.

Read more at: http://www.tu-chemnitz.de/phil/english/chairs/linguist/real/independent/llc/Conference1998/Papers/Leech/Leech.htm