This should be a proposal
I believe this should be written as a proposal rather than declaring this as the preferred way, as this would be a major change for data consumers. And I would say that using unit=* is inadequate, for example a road might have a width=*, maxspeed=*, and maxheight=*, all of which could use different units. Perhaps better would be to use maxspeed:unit=*, width:unit=*, etc. -- Joshdoe 15:21, 27 July 2011 (BST)
It is better for data handling to have the unit separated out from the value of a key.
So let the consumer of the data separate it if they wish in their pre-processing phase, rather than try and change it in the OSM database which needs you to convince thousands of mappers to learn a new way of tagging for no real benefit. No-one seems to have a problem at the moment; see for example the various ITO Map speed limit layers --EdLoach 19:13, 27 July 2011 (BST)