Could an illegal immigrant who married a US citizen be deported if he files the i-485 and the INS sees he's illegal?

It depends on how the immigrant entered the U.S. If they entered legally, then no they do not have to leave the country. However, if they entered ilegally, they would have to their country of origin. What will happen here: Since you filed an i-485, I imagine that you have a lawyer (stupid not to have one). After a couple of packets, you will receive a visa, and an appointment with the Immigration in your country. You will then have to go to your country, and report to them. You wife/husband will need to show how it would cause them hardship for you not to be here (think pshycologist papers,schooling, children, etc.). If you have not had any major problems here in the US, They will then grant you a visa to get back, and your residency. Hope this helps. This is how I am getting it done. I know that it's a burden to have to go back, but think of it this way... chance to meet the in-laws. LOL