I do not have a single.php, and if it is coming from a plugin, I cannot tell which. I disabled the Buzzup plugin as well as the one that inserts Digg (diggclick)... but that did not remove the "Email this" link.
Interestingly, I would think it has to do with Stumble it (that used to be in the spot where "Email this" appears. For the life of me, I cannot find the way I inserted the previous Stumble It link! (It is not in the theme's index.php, and I don't have a single.php where it normally would go.) I am beginning to think that maybe at one time I had a Sociable plugin, and enabled Stumble it throught that... but Sociable does not appear in my list of either enabled or not enabled plugins.
UPDATE: Looking at Feedburner, in their "feed flare" service, "email this" was all of a sudden checked, and that was also the source of the Stumble It link that was missing all of a sudden. I will play with that to disable it (which is not yet working).
ALSO: How do I enable "fancy permalinks" in 2.7 to make Super Cache work correctly?