A small function to get the escaped value of a named cookie. There's also a bigger function that works as a cookie setting preparer, though it does only get the escaped value and does not yet fit into 140 bytes at the moment (2 bytes golfing, anyone?):
var C=function(c,d,e){e='';for(d in c)c.hasOwnProperty(d)&&(e+=(e?'; ':e)+d+'='+c[d]);return''+c!==c?e:(document.cookie.match(c+'=(.+?);')||0)[1]}
// setting a Cookie:
document.cookie = C({cookiename: 'testcookie', expires: (new Date(new Date()*1+6E10)).toGMTString()});
C('cookiename') // -> returns the still escaped value of the Cookie "cookiename"
I made a habit of writing functions with less than 4 arguments, since they can be confused so easily otherwise. A colleague of mine wrote a cookie setter function that had up to 6 arguments and always had to look up the correct order; whereas the properties of an input object can be easily spotted by its keys.
Your pattern has its merits, anyway. Usually I try to create an interface which is usable in the most obvious ways (so the "perfect" cookie function would allow for C(key, value[, object]) too), but haven't yet found a way to do so within 140 bytes.