We ran into the same problem. We applied for our son’s SS card with his status as “legal alien.” We did this because his finalization was on 12/10/09, and we worried that if we waited for a passport or citizenship certificate, we wouldn’t have time to get the SS card by April 15th. We waited about 8 weeks and went back to the SS office to inquire about the card. Apparently, when you file for a SS card as a “legal alien” USCIS must validate all supporting documents. So even though we had Manu’s greed card with us, the SS office has to wait for USCIS to confirm it is indeed a real card. Well, for whatever reason, they are dragging their feet, and we still don’t have the SS number. Like you, we submitted the N-600 to obtain a temporary tax ID number. Surprisingly that showed up in only about 3 weeks, and just last weekend I finished our taxes with that number. My understanding is that, if for some reason you do not get it in time, you’ll need to either file for an extension, or file as normal and then file an amended return once you get the SS number. I think you’ll get the number in time though.
We have since applied for Manu’s passport as a means of proving citizenship. We’ll apply for the USCIS certificate after we get our refund.