An interesting item came across my email today. It seems that if you set the SkipLabel property to True you get the side effect of disabling the association of workitems with those builds.
The reason behind this is that the work item association feature will compare the LastGoodLabel property (applied after each successful build) on the build definition with the current build label to determine the changesets that went into this build. No label, no changeset list.
On the plus side, you really shouldn’t be associating changesets with builds that can never be recreated anyway so this is really a good thing.
Thanks to Martin Woodward for the tidbit.