Last week, Dervish noted a speech by an Australian government official who demanded Muslims who move to Australia 'respect Australian law and give up the shari'ah'. More recently, in Britain, Trevor Phillips, the chairman of the Commission for Racial Equality (CRE) demanded that Muslims who want 'Islamic shari'ah law' should 'leave Britain'.
The problem with both calls is they're largely misinformed. Shari'ah exists where ever Muslims happen to exist. So if a Muslim decides not to eat a bacon sandwich, to avoid alcohol, to visit the mosque on a Friday, to perform the qui-daily pray, to pay zakat, to ritually wash herself, and the Muslim does all this living in London, New York or Sydney, then shari'ah is in existence and being observed.
It should be realised that the shari'ah is not a single text that is opened and from which one reads an answer to a particular situation. To talk of "shari'ah law" is really to talk of the cumulative interpretation of a whole range of texts and customs, foremost of which are the Qur'an and hadith material.  It can be seen as a combination of practice, theory, belief and custom. A social history of Muslims could be written using these interpretations as the shari'ah has developed depending on the situations (cultural/geographical/social) in which Muslims have found, and continue to find, themselves. It does violence to the concept of shari'ah to reduce it simply to a set of 'laws' to be enforced by the state, which is what is assumed by far too many people, including Muslims. When Western Muslims talk about shari'ah they are not likely to be referring to the functions of the state or seditious activites which undermine the state or society at large; but about individual matters of conscience, piety and observance of religious duties. If anything, authoritative shari'ah interpretations require Muslims to obey the local laws in their place of residence, unless they feel the laws force them to contradict their beliefs, in which case it is widely held opinion that the Muslim should try to leave for another place. Of course, this doesn't mean state laws are not based upon shari'ah. 
The point of dispute (if one is to recognise the dispute) between 'reformists', 'traditionalists', 'modernists' and 'progressives' (and so on) is to how to interpret the shari'ah, not merely that something called shari'ah exists; it always exists as it is a fundamental concept of Islam.
 Scholarly consensus and local custom can also be taken a source of shari'ah. Another perspective would argue that the shari'ah is only the core sources of Islam, the Qur'an and sunna, and everything else on top of that is reduced to human interpretation. Literally, the word means 'the way to the watering hole'.
 The word qanun is sometimes used to refer to state (positive) law.