What external systems is Bitcoin commerce dependent on? If there was a concerted effort to "shut it down", where are the vulnerabilities?

The potential for the Bitcoin client to communicate with other Bitcoin nodes has been discussed at length, but what about the other parts of the Bitcoin ecosystem that have we become dependent upon?  If they disappear (e.g., denial of service, or taken down), will Bitcoin continue to function as a currency and payment system?

What are the most important systems that we are dependent on and how are we affected as a result if services from these systems end?

For an example of one such service -- what would be the effect if BlockExplorer.com were to be forced offline.
Whereas Block Explorer might be a useful tool, Bitcoin could survive without it.

Most important parts of the Bitcoin ecosystem, asides the main client that is, are the exchanges. They give the currency its value and without them, Bitcoin would become a fringe technology (no merchants would deal in it, as they couldn't pay their taxes with it.

Then we have the mining software and to a lesser degree the hardware (FPGA boards and the like). Should those be gone or less available it would be easier to launch a 51% attack on the network.

After that there are the mining pools. If they would disappear the miners would have a harder time earning a predictable wage. This dependancy has been mitigated with P2Pool though, so the impact is smaller than it was earlier.

After that the services are less essential. Losing the Bitcoin Forum, Wiki, SE, WeUseCoins or the like might make it harder for new people to get into Bitcoin, but wouldn't make it less functional.