You don't mention any dependents so I mention this only as an option - if you didn't live with your spouse for more than the last 6 months of the year and you paid more than half of the cost of keeping up your home during the year and your home was the main home of your dependent child during that time you are eligible to file as head-of-household for the year.
If you are filling out form 8958 in a community property state then half of your ex's tax bill would fall to you anyway. Everything is evenly divided on form 8958. However, if you feel like you did not benefit from an item of community property then you can disregard it (ie. leave it off of your return). Go here to look at IRS publication 555 and click on the section marked "Community Property Laws Disregarded".
Lastly, if you can't get your ex's social security number in order to file a married-filing-separate return then you will have to paper file the tax return. Enter all the information that you have for your ex and then write "UNAVAILABLE" where the social security number goes.
The second that your divorce is final you file as single so as long as it is final before December 31st of this year then you will not have this problem for next year.
@johnrhyde
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