When I posted this question the version of JQuery that I was using didn't include an isArray function. If it had have I would have probably just used it trusting that implementation to be the best browser independant way to perform this particular type check.
Since JQuery now does offer this function, I would always use it...
$.isArray(obj);
(as of version 1.6.2) It is still implemented using comparisons on strings in the form
toString.call(obj) === "[object Array]"
