While agave itself is a low GL carb, and described by Barry Sears as a good addition to the Zone (refer to his comments about agave in the archives of drsears.com), IMO, yes, consumed in the form of nectar, it would be unfavorable. I'd consider agave nectar to be comparable to the juice of a favorable fruit or veggie, favorable in it's original form, unfavorable in it's processed form. Once you process the original food, breaking it down to isolate the insulin stimulating liquid, that resulting product, be it nectar or juice, is considered unfavorable carb for Zone purposes(eg. apples, vs apple juice). In Zone books, you can read more about how processing a favorable carb renders it unfavorable. Oats are another excellent example. Steel cut oats are favorable, but rolled oats and oat flour (more processed) are unfavorable.
Fructose, which Barry describes as one of his preferred sweeteners, and which is much less stimulating to insulin than sugar, is also still considered to be an unfavorable carb.