急問：等綠卡沒到，卻收到welcome notice?

我們於12月9日持廣州移民簽證（E11和E14)從紐約入境，當時在入境處按了右手食指印，按一般規律應該1個多月能收到正式卡片，可是昨天又收到一封信（跟前天一樣的內容，就是notice day 為29日），內容如下：

Notice Type: Welcome Notice
receipt number SRC-**-**-****
case type OS155A immigrant visa and alien registration
Applicant A*** *** *** NAME
notice day Dec. 23,2009

welcome to U.S. We have received your immigrant visa packet from the port of entry where you entered U.S. At the top of this notice you will see a very important number (A-number).This account number is very important to you.you will need it whenever you contact us. The receipt number shown above is simply a tracking number for your immigrant visa packet.

you should hear from us within 30 days from the day shown on this notce....

Note if you have submitted a SB1 visa, you will not receive a new I-551 card. Your previous A-number and the corresponding I-551 card are still valid unless the card expired,......

immigrant & naturalization service TEXAS SERVICE CENTER

我在網上查我的case status 顯示為：
Post-Decision Activity
On January 3, 2010, we mailed you a notice that we have approved this OS155A IMMIGRANT VISA AND ALIEN REGISTRATION. Please follow any instructions on the notice. If you move before you receive the notice, call customer service at 1-800-375-5283.

問題是我準備1月9日回中國，在這之前不一定能收到他的信啊，真不知道該咋辦？

請問什麽是OS155A呀？會讓我們做什麽呢？會不會需要人呆在美國境內做些什麽呢？

多謝！