I've already posted elsewhere that I feel that Canada [because it's a western country,
it's vulnerable, and negligent] maybe the "next target" for Al Qaeda after Spain;
who was ever willing to read my opinion.

But back to your topic, Dave!...
Al Qaeda smuggling nerve-agents into the USA maybe extremely difficult as you say,
but that doesn't preclude them from using local resources within the target country.
(1) If a terrorist wants to obtain/manufacture explosives?...Steal large supplies
of nitric-acid from metal-etching firms, or steal dynamite from mining companies, etc...
(2) If a terrorist wants to obtain/manufacture nerve-agents?
Steal or purchase chemical supplies related it "ink manufacturing", and supplies of
insecticide chemicals, and alcohol.

Or, if the Al Qaeda terrorists are in France or Belgium, by simply driving a minibus or van
into the former WW1 & WW2 battle-sites near Verdun, Passendaele, Flanders
with a handy metal-detector and a spade they could literally dig-up numerous UNEXPLODED
SHELLS from those wars [loaded with kilos, even tens of kilos of high-explosive]. And if those shells dug up give up the sounds of SLOSHING?...they're projectiles containing mustard-gas, phosgene, etc.
To a desperate terrorist-cell seeking to cause major carnage in a crowded city plaza, they'll do.