Eric-
Is this a request to testify as an expert? If so, you are not required to do so. And, if you are not comfortable with doing so, I'd advise against it.
If you are being "compelled" to testify by virtue of a subpoena, then you will have to go. You will be expected to answer the questions as they relate to your appraisal report. Remember, either as an expert or under compulsion, you can only advocate your opinion on the report; you are not there to help one party or another.
On a broader issue, when you did this assignment, are you sure it is USPAP compliant?
Don't get offended; if you read this forum often enough, you'll see that many appraisers who have significant experience in mortgage finance transaction appraisals have very little experience in non-lender appraisals, and consequently fail to apply USPAP in a way that is required for the intended use, intended user and SOW for the assignment.
If you wrote your report in the same/similar manner that you would for a mortgage finance transaction, a sharp attorney (or, a not so sharp one with a competent appraiser as a consultant) will eat you alive (IMO).
If any of these possibilities concern you, I suggest you:
1. Explain to the client that court testimony was not part of the assignment agreement, and that you opt not to testify. Plan on your client not being happy to hear that. Especially if the fact that the assignment was for a divorce and could end up in court was discussed at sometime in the past.
2. Have your appraisal competently reviewed; this way you'll know what someone else could "zero" in on.
3. Spend a few days in court to see how the proceedings go and see how people testify. At a minimum, this will increase your comfort level (court proceedings won't be a complete "unknown" to you).
4. If your client is working with an attorney, explain to the attorney your position and hesitancy in testifying. The attorney could decide the best thing to do is not to call you, and you are off the hook.
Good luck!