Let’s look at the passage itself, in a historical-grammatical context.  Peter is writing to confront and expose false teachers that were infiltrating the church.  He begins the book by reminding them of the truth of their salvation and their responsibility to grow in the faith (1:1-15).  He discusses the trustworthiness of Scripture because of its supernatural source (1:16-21).

Starting in chapter 2, he confronts the false teachers and sets up a “them” vs. “you” motif that he’s going to continue through the rest of the book.  Moving through chapter 2, he continually pronounces condemnation on these false teachers.  Verses 12-17:  “But these, like natural brute beasts made to be caught and destroyed, speak evil of the things they do not understand, and will utterly perish in their own corruption, and will receive the wages of unrighteousness. . . . These are wells without water, clouds carried by a tempest, for whom is reserved the blackness of darkness forever.”  Pretty strong language.

Chapter 3 begins with Peter shifting gears to speak more directly to the elect who were apparently becoming discouraged by the seeming delayed appearance of Christ.  Some scoffers had entered the church who were putting forward an emotional argument saying, “Where is the promise of His coming?” (3:4).  They were denying Christ by affirming a form of uniformitarianism.  In verse 8, we see Peter again switching gears from warning about scoffers to encouraging the “beloved”.

Verse 8 states:  “But, beloved, do not forget this one thing, that with the Lord one day is as a thousand years, and a thousand years as one day.”  Peter is building up an argument where he is going to show them that God is not confined to their timing, but operates on His own.  And also that this idea should not cause them to become discouraged, thinking that God is slow in keeping His promises.

Then we get to the pivotal verse:  “The Lord is not slack concerning His promise, as some count slackness, but is longsuffering toward us, not willing that any should perish but that all should come to repentance.”  So, the “beloved” are to be comforted by the fact that God is not slow in fulfilling His promises, as some people are accusing Him.  But, rather, He is patient.  Who is He patient toward?  The verse states, “us”.  The immediate context seems to confine “us” to the “beloved” and Peter.  In fact, the whole time Peter has set up “them” as being the scoffers and false teachers.  The “us” is set up as opposite to “them”.  So why is God patient toward “us” or the “beloved”?  He is patient because He is not willing that any (of “us”) should perish, but that all (of “us”) should come to repentance.

Peter’s argument here is that Christ’s coming in judgment is delayed because of His patient desire to see “all” come to repentance.  If all is including those that will never come to faith, then He will have to delay His judgment indefinitely because of His desire for “them” to come to repentance.  But if all is a set number of elect chosen from the foundation of the world, then God’s patience will eventually be satisfied when “all” finally do come to repentance.

Here’s MacArthur on the passage:

The “any” must refer to those whom the Lord has chosen and will call to complete the redeemed, i.e., the “us.”  Since the whole passage is about God’s destroying the wicked, His patience is not so He can save all of them, but so that He can receive all His own.  He can’t be waiting for everyone to be saved, since the emphasis is that He will destroy the world and the ungodly. . . .  “All” (cf. “us,” “any”) must refer to all who are God’s people who will come to Christ to make up the full number of the people of God.  The reason for the delay in Christ’s coming and the attendant judgments is not because He is slow to keep His promise, or because He wants to judge more of the wicked, or because He is impotent in the face of wickedness.  He delays His coming because He is patient and desires the time for His people to repent.

So there’s an interpretation of 2 Peter 3:9.  Sorry my Greek is not up to snuff, but I don’t think appealing to nuances of the Greek will change the interpretation here.  Anyone is welcome to throw down in the combox, just try to avoid silly arguments like, “Of course you interpret it that way, you’re a Calvinist.”  Show me my error based on the historical-grammatical method.