Just as this week's Parsha describes the final stages of the construction of the Mishkan, this week's Haftara describes the final stages of the construction of the First Bet HaMikdash.
In both cases, the climax centers on the Sh'china - in the form of the anan - covering the new structure, a sign that indeed His Divine Presence rests within Am Yisrael. [See Shmot 40:35; compare I Melachim 8:11.]
The Return of the Aron
The Haftara begins with the description of how the Aron was brought into the Mikdash. Recall that the Aron, unlike the Shulchan and Menorah, is not 'kli' ('vessel') of the Mikdash. While all of the other keilim have a daily or weekly ritual that is performed upon them, the Aron has none. The Aron is not part of the Mishkan; rather the Mishkan (and/or Mikdash) is the proper place to keep the Aron. Ideally, it should be kept in the Kodesh K'doshim; however, the Mikdash can function without it (as was the case in Bayit Sheni).
For example, we find many instances when the Aron is taken to battle or leads Bnei Yisrael as they travel (see Yehoshua 3:3-4 and Bamidbar 10:33-36). This obviously is not done with any of the other keilim of the Mikdash.
Furthermore, since the destruction of Shilo (some one hundred years earlier, during the time of Eli and Shmuel), the Aron had actually been separated from the Mishkan. Even though the Mishkan had been rebuilt in Nov and later moved to Givon, the Aron never returned to it. Instead, it remained in Kiryat Ye'arim on the western border of the tribe of Binyamin. For some reason, the Aron and the Mishkan never reunited until Shlomo built the Temple.
Hence, it is not surprising that Sefer Melachim focuses on the rejoicing that takes place when the Aron is finally brought from the City of David into the Bet HaMikdash (see 8:1-11). The Mikdash is now complete.
New Keruvim (with their feet on the floor)
The Aron is placed in the new Kodesh K'doshim under two new Keruvim that are very different than the Keruvim that covered the Aron in the Mishkan (8:6; see also 6:23-35). Unlike the Keruvim in the Mishkan, which were located on the Kaporet, the feet of the new Keruvim are firmly attached to the floor of the Mikdash. This may reflect the permnent nature of the Mikdash - as opposed to the temporary nature of the Mishkan.
The second half of the Haftara (8:12-21) records Shlomo's special tefilla in which he thanks God for fulfilling His promise to David that his son would build Mikdash. [See II Shmuel 7:1-15.]
Note how Shlomo refers to the purpose of the Mikdash - "a house dedicated to God's Name" (8:17 and 8:20). Now that God's Temple is finally constructed, for the first time since Yetziyat Mitzrayim (see 8:16 and 8:21) Am Yisrael is ready to fulfill its Divine destiny - to bring God's Name to all nations (as explained in our shiurim on Sefer Breishit). Am Yisrael has finally achieved a level where they have gained the respect of other nations.
This same theme is reflected later in Shlomo's tefillah when he explains that this new House for God is also open for the prayers of the non-Jew:
"And even for the non-Jew, who is not from your people, who may come from a distant land for the sake of Your name, for he has heard how great Your Name is... and he comes to pray in this House. You should listen to his prayer in order that all the nations shall know Your Name, and learn to fear You just as Your nation Yisrael does, for Your Name is identified with this House which I have built." (8:41-43)