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The USSR refined enough of both for thousands of devices. These materials are now guarded by starving lice ridden Russian soldiers too young/malnourished to shave. While this is too-frighteningly true, what Russian soldier's going to willingly sell this stuff to a country that poses as much a threat to them as to the US (hint, a Moscow-sanctioned sale is more likely than this)?

A state-to-state sale is the likeliest method of Iraq getting enough fast-fissionable uranium. Even then, it'll likely not be pure enough to be just dumped into a weapon. That's where refining comes in. Little Boy's design was so straightforward it was never tested. No timing is required unless you want to add neutron triggering for more yield. 10-20 ktons is pretty much a "firecracker" by modern standards but is more than enough to jack up an urban area. No missle required, thousands of cargo containers enter the US everyday from around the world. If we can't even get a pea shooter in a cockpit in 365 days, how long will it take to get Geiger counters at all of our ports.

And it was so bulky that the largest aircraft in WWII (the B-29) could only carry one 20kT device. A MiG might be able to carry one, but certainly no Scud (and if that particular design could be miniaturized enough to fit on top of a missile, it likely would detonate prematurely due to the physics of launch). The physic and size limitations is why everyone ultimately went with the much-more-complex Fat Man.

As for how a nuke would get from Iraq to the US, you just nailed it. There's no way that even if the Iraqis had a 747 (or other transport aircraft that theoretically would have the range to make it from Iraq to the US), it would make it too far out of Iraq.

Bio's are simpler, more available, and more dangerous. I wouldn't be surprised by the democratization of u-235 guns as well.

Again, you nailed it on the bios. Sure they're not as immediately deadly, but that lack of immediateness would work in the favor of a terrorist that wasn't looking for a quick trip to his 72 Janet Renos.

The Little Boy bomb is, at best, a stepping stone. About its only usefulness is a truck/ship bomb, and not a particularily-powerful one at that. In fact, I'd be a bit more worried about a radiological bomb than that (again, less fingerprints leading back to the terrorist).