[quote]Never legally separated. Long story, STBX filed for divorce in November 2006, but we reconciled. She moved out in Ocotber 2007 for good. Am told that October 2007 is our "date of separation" for divorce purposes. [/quote]

The tax bill is a marital debt (or at least a portion of it). The IRS doesn't care if the separation date was 10/07. All they care about is your marital status as of 12/31/07. And that was married.

And even for divorce purposes, some of the tax bill for 2007 is going to be considered joint as you were living together for most of 2007.