<p>I have some commits that I've decided, after the fact, are going to be more branch work then trunk work. How do I create the branch and revert the trunk while still ensuring merging isn't painful later?</p>

<p>Is it as simple as copying the current trunk to a branch and reverting the trunk? Or will this create headaches later?</p>

There's nothing wrong with following Philip's method, other than it leaves some "cruft" in the revision history. If you wanted to removed them for tidiness sake, and the revisions are at HEAD you could remove them from the repository by following
these instructions
.
Update:
Philip's method is better than the one suggested in the question for the reasons he stated. Mine and Philip's methods would be similar, except that insead of reverting the trunk I propose removing the revisions from the revision history. (as I said, this can only be done if all the revisions you want to remove are at the HEAD of the repository.)