<p>I haven't been able to find any documentation on hardware access via the iPhone SDK so far. I'd like to be able to send signals via the dock connector to an external hardware device but haven't seen any evidence that this is accessible via the SDK (not interested in possibilities on jailbroken iPhones).</p>

<p>Anyone have any pointers to docs for this or some idea of what deep dark corner i should look?</p>

This falls into the range of capabilities that requires working with Apple to get a special license.  At, I presume, a special price.
This has changed to some extent with the 3.0 version of the iPhone firmware.  If PyjamaSam is correct there is still some special activity required to get the connector specifications.