An amazing vort from R' Shmuel Birenbaum: among the items collected for the Mishkan were precious stones to be used in the choshen. The Torah calls these stones "avnei milu'im" (25:7) because, as Rashi explains, they were used to fill the holes that they were set in. Why would the Torah focus on the fact that these stones served as filler instead of focusing on their inherent precious value and beauty? We see from here, answered R' S Birenbaum, that true value comes from being able to make something, or someone, whole.
The idea I have below is admittedly half-baked and probably needs some work.
The gemara (Kesubos 62) writes that when Rebbi's son was getting engaged, he made a deal with the in-laws that he would learn for 12 years before the wedding. The kallah then passed in front of the chassan, and after he saw her, he said, "How about we make it 6 years?" The kallah came by again, and he said, "How about we first get married and then I go learn?' He was a little embarrassed by the change of heart, but the shverr said to him not to be concerned, as Hashem did the same thing. When we crossed Yam Suf, the plan was "tivi'eimo v'sitaeimo b'har nachalascha," that we will come to Eretz Yisrael, and only then "mikdash Hashem konenu yadecha," we would get a Mikdash. However, as we read in our parsha, Hashem decided to speed things up and give us a Mishkan while we were in the midbar.
Hashem doesn't change his mind like an anxious chassan. What was the hava amina here and what's the maskana?
The Yismach Moshe quotes a Midrash that writes that when Klal Yisrael said na'aseh v'nishma, Hashem responded by giving them the command of "v'yikchu li terumah." What's the connection? According to Ramban, the revelation of Torah that took place at Sinai continued in the Mishkan, and we find many parallels between the two. Just as people had to keep their distance from Sinai, so too, no one could enter the Mishkan if not for the sake of avodah. Moshe was called to ascend Sinai; Moshe heard Hashem calling to him from the Mishkan. The Midrash opens our parsha with a warning of "ki lekach tov nasati lachem torasi al ta'azovu" -- the purpose of the Mishkan was to be a Torah center. Therefore, Hashem originally presented the plan to Klal Yisrael for the Mishkan to be built only after they had time to spend immersed in learning Torah, growing in their love for Torah. By the time they reached Eretz Yisrael they would be ready. Klal Yisrael, however, rose to the occasion, and by saying "na'aseh v'nishma" they demonstrated that they loved Torah already -- there was no need to wait.
I would suggest a slightly different twist. The gemara in Brachos writes that R' Yochanan wondered how there could possibly be old people in Bavel when the Torah promises that the bracha of "l'ma'an yirbu y'meichem," of long life, would be given "al ha'adamah asher nishba Hashem l'avoseichem," only in Eretz Yisrael. The gemara answers that those people blessed with long life earned that reward because they came to shul on time for davening. How does that answer the question -- bottom line, they are not living in Eretz Yisrael? The meforshim quote another gemara that teaches that the shuls and batei midrash of chutz la'aretz will, in the time of geulah, be transported to Eretz Yisrael. In other words, the shuls and batei midrash of Bavel were, in potential, "al ha'adamah asher nishba Hashem l'avoseichem." Just like when you are in a foreign country, if you enter the embassy of your home country, you are on home soil, so too, when one enters a shul or beis midrash one is breathing "avira d'Eretz Yisrael."
Perhaps this is what the gemara in Kesubos means. Originally Hashem presented the plan for the Mikdash as something that could be built only in Eretz Yisrael, not simply, like the Yismach Moshe suggests, as a delay tactic to give people time to acquire a love ofTorah, but because "KI mi'Tzion teitzei Torah" -- it requires geographically being in Eretz Yisrael to truly absorb Torah. However, when Klal Yisrael declared "na'aseh v'nishma," the proved that like the old people of Bavel, a taste of the "avira d'Eretz Yisrael" could be felt in chutz la'aretz as well, and Torah could find a home in an "embassy" there.