I'm sad this has happened, but I have to say I'm not surprised. I remember suggesting to you that you likely could have a problem since you were getting new insurance, and had been treated for the condition before this new insurance went into effect. You were being treated for the symptoms, and likely the doctor had some diagnosis involving your gall bladder in your records even before the confirming scan. The scan may actually have served to confirm that this was a pre-existing condition because it was pieced together with previous reports from your doctor.
I'm no defender of health insurance companies, that's for sure, but the devil is in the details with them. And all of us, whether we like it or not, are ultimately responsible for paying our costs if our insurance doesn't pay. It'll be in the fine print at your doctor's office, the hospital, and the insurance contracts. Although it seems doctors are responsible for this, in reality it's all done as a service as doctors know most patients wouldn't follow through on the process of pre-certification and submitting claim forms, and thus the doctor or hospital would end up never getting paid, so they take it on as an extra service. I can say this because I've been on both sides of the fence, and the payments from insurance companies were, in essence, my pay check.
If you got something in writing that has a pre-certification number, especially if it's after the scan, you may have a leg to stand on in an appeal. Also you do need to see what your doctor's office did re: pre-certification, and if they got the approval after you had the test. I'm saying after the test because the test confirmed the diagnosis, but the preliminary diagnosis was made before you got this new insurance. If they pre-approved it after the test results were in, then the insurance co. should have had information from your doctor's records and been able to decide it was a pre-existing condition, and deny the approval. Unfortunately, while your doctor may have said he'd taken care of it, they usually have a person (or an entire staff) dedicated to doing these things, and he likely delegated it to them.
I do think the suggestion others have made about talking with the hospital, in addition to trying to appeal the insurance denial, about their charity program and what the income limits are for that is very worthwhile. Of our two major hospitals in my region, one will write off care if your income is below 200% of the federal poverty level, but the other (which has a religious affiliation) will write off care if your income is below 300%. So there can be quite a variation between hospitals.
The new health insurance reform is supposed to provide for a pool that adults with pre-existing conditions can buy into who can't otherwise get insurance until the rest of the changes go into effect in 2014. How expensive that coverage will be, though, I doubt has been determined. It will probably be some type of buy into one of the government programs, but that's better than nothing.
I know this isn't what you wanted to read, but I'm just trying to help you the best I can. And you know ultimately, you and your husband can only do what you can do. That's the attitude that keeps me somewhat sane these days regarding finances; I've had to let lots of things fall by the wayside, including what used to be an excellent credit rating. Sometimes survival takes precedence.
But at any rate it's going to be a long process, so I'd make a few phone calls to your doctor's office and the hospital and your insurance, but try to still focus on your healing. I know that's easier said than done.
Hope this helps a bit.