Is the order objects are return by a jQuery selector specified?

Yes.
The jQuery 1.3.2 release notes say:

Elements Returned in Document Order

This is a change to jQuery's selector engine that re-orders the returned results to be in document order, instead of the order in which the selectors were passed in. This change was done in order to be in compliance with the Selectors API specification (which jQuery uses, internally, in browsers that support it).

This wasn't the case on jQuery 1.3:

The order of "a, b, c" style selectors may change. Browsers that support querySelectorAll (Safari, Firefox 3.5+, Opera 10+, IE 8+) will return the elements in document order, other browsers will (currently) return them in the order specified. In 1.3.2 and later release all comma-separated selectors will be returned in document order.

There may be some exceptions, for example from the prevAll() documentation:

"Given a jQuery object that represents a set of DOM elements, the .prevAll() method searches through the predecessors of these elements in the DOM tree and construct a new jQuery object from the matching elements; the elements are returned in order beginning with the closest sibling."

http://api.jquery.com/prevAll/

Yes, they're always in the order as they're in the DOM.