The following applies to git 1.6.4.
I did a "git log -S" with some search term, and it filtered out seven disconnected commits from a fairly large repository.
Then I tried "gitk -S" with the same term. In the upper pane I briefly see the seven commits, each with a dot in a different column and a down-pointing arrow. After a while (presumably when it has searched through the whole repo and has discovered that the commits are disconnected), the display is updated to include the preceding commits as white dots. However, the total number of shown commits is still restricted to seven, which means that I now only see the first four of the commits I should see.