This Shabbat -- the one right after Pesach is when many Jews bake "Shlissel Challah" -- Challah in the shape of a key (or a key baked inside the Challah). It's considered a segula for all sorts of good things...
Orthomom (remember her?) found all the sources last year:
1. Based on "Pitchi Li Achoti, Ra'ayati..." ("Open up, my darling..."--Shir HaShirim 5:2), on which the Medrash states "Pitchu li petach ke-chudo shel machat...," (cf. Shi HaShirim Rabbah 5, s.v. "Kol Dodi Dofek") = something like "Open your hearts (in teshuvah) like the eye of the needle, and I (God) will open the rest like [a very largeJosh Waxman in the parsha blog also had more on the subject.
2. According to Kabbalah on Pesach the gates to heaven were open, and following Pesach the lower gates are shut, and it's up to us to open them again, therefor on the 1st Shabbat we put the key on the challah to show that through the mitzvah of Shabbat we are opening the locks [original source?].
3. In the desert the Jewish people ate from the manna until after Pesach upon entering the land (with the bringing of the Omer, see: Josh. 5:11), at which point the ate from the produce of the land, and became dependant on their livelihood for the first time (now they had no manna). The key in the challah after Pesach is a request the God should open the Sha'arei Parnasah (gates of livelihood). Alternatively, the manna began to fall in the month of Iyyar, and this Shabbat is always Shabbat Mevarchim Iyyar.
See: Sefer Ta'amei HaMinhagim, pp. 249-50.
See: Encyclopedia Judaica, vol. 6, pp. 1419-20 for a photo of a shluss challah (and other "special" challot). It seems (from both of the above sources) that the minhag was to bake the key on top of the challah not inside (a la the old jail break trick).
But the source I'm familar with is from the Sefer Ta'amei Minhagim that says it's based on the pasuk, 'יפתח ה' לך את אוצרו הטוב את השמים לתת מטר ארצך בעתו'
PS: Hat-tip to CK from Jewlicious for the Challah Hu Akbar graphic (apparently :)
Wherever I am, my blog turns towards Eretz Yisrael טובה הארץ מאד מאד