May (pres)/might (past) is a modal verb (it modifies the verb it is being used with).

The dictionary says it expresses possibility, permission, a wish or a hope (e.g., may it be so) - Concise OED, 11th edn.

It adds in an explanatory note that both may and might are used interchangeably - I may go and I might go have the same meaning - whereas traditionalists insist that only may would be correct in that sense, while I may have gone would be an incorrect use of the past tense, I might have gone being the correct version.

The "modern" usage supposedly allows for different shades of meaning, but if the way the words are used is controversial, how can the subtle differences be distinguished? Is may more or less permissive than might?

Also, which do you prefer, double quotation marks or single?