Aaron asks, "I have just received the I-551 stamp in my passport and due to a family situation I must leave immediately to my native country. I still do not have the plastic green card in my hand. Is it safe for me to travel overseas and be able to return to the US without any problem?"
Actually, the I551 stamp is proof of your being a permanent resident of the United States. While it is required that you show your green card to the immigration agent in order to enter the country, since it has been so recent that you have been approved, you will be fine. The agent at the airport will be able to see what date you were approved. Maybe there could be a few extra questions but that's about it. For even greater peace of mind, you can carry the approval letter from USCIS in your paperwork to show that you have been approved.