I got a letter from the IRS. What does this mean?

[TheGrasshopper]

Dear Taxpayer:

We received your Dec. 31, 2006, Form 1040 federal individual income tax return, but we need more information to process the return accurately. Please send us this letter with your reply in the enclosed envelope within 20 days from the date of this letter.

Please enclose only the information requested and any forms, schedules or other information required to support your entries. Please note the inclusion of two copies of any requested forms and/or schedules and return only one of each applicable form(s) and/or schedule(s) with your reply. The second copy is for you to keep for your records. DO NOT SEND A COPY OF YOUR RETURN, unless specifically instructed to do so. We will issue any refund due to you in about 6 to 8 weeks from the time we receive your response. Since we can support them, we may have to increase the tax you owe or reduce your refund if we do not hear from you.

Your Form 1040 doesn't show your original signature(s). Please sign the declaration at the end of this letter and note the additional requirements that may apply to you.

1.) If this is a joint return, both husband and wife must sign the declaration.
2.) If you cna't write your name, please sign your mark in the presence of two witnesses. The signatures of the witnesses are also required.
3.) Doesn't apply to me
4.) Doesn't apply.

OK so my question is this. Do they just want my signature or is there something I'm not seeing? Sorry for the long post, I'm just confused because I reported everything accurately.