Everything went fine with the actual immigration procedures. I'm a US citizen by birth, and I've been married to my Canadian-English wife for two years. Hold your congratulations--we separated not long after her provisional green card arrived in summer 2006.
However, she stalled with the divorce proceedings for so long that now it's not certain whether the divorce will be final by the time she needs to file to remove her green card's provisional status. She wants me to file the I-751 jointly as a married couple, and then she wants to file a new set of papers afterward when the divorce is official.
There's a letter dated August 2006 on DHS's record that we had separated and were planning divorce. The I-751 has checkboxes for "married, filing together" and "divorced" and "battered spouse" (to oversimplify), but none for "separated: divorce in progress." While I don't want to make her life difficult, I feel it would be a pretense, a perjury by omission, to sign jointly as a married couple.
I'm researching the issue now, and may decide to seek legal advice locally. In the meantime, my heart would be cheered by any words of wisdom or experience. Thanks!