Itadakimasu is rather the kenjougo of moraimasu.
Kenjougo is the way of speaking politely/humbly about yourself. (Contrary sonkeigo is when you speak politely to an other person.).
Moraimasu (morau in jisho-kei) is the verb receive.
Of course it's often just something you say before eating, but if you speak with a clerk in a store or with some other polite person it probably WON'T mean he or she will start eating
There are acually other examples like this, where "things you just say" acually just are polite counterparts of verbs, such as oyasumi nasai (good night), but that's another story
(ofc not 100% sure about all this, haven't spoken japanese for a while)
This post was edited by Vashan on Sep 9 2008 12:50pm