If you do not already do it, First check the status of your application the this website:
https://egov.immigration.gov/cris/caseStatusSearchDisplay.do
Enter your application reciept number. If the status message does not say anything about the card or welcome letter, but just about I-485 approval then you need to contact them.
If you have your I-485 approval letter, call them immediately on 1-800-375-5283. Follow the prompts and you will be connnected to a service rep(after a long wait) tell your case to the officer and if everything is OK (if you have not moved to a new house(s) in the past few years, etc) your application should be expedited.
I'll tell you my experience about the call I made yesterday:
Mine & my wife's I-485 got approved in end-of-Jan-2006 and after that there was no activity. Out of furstration, I called the above number and waited for 15-20 minutes before a cust service rep came on line. I told her my case and she asked me to hold while she forwarded my call to an immigration officer. So I was on hold for another 35-40 minutes when the officer came online. I narrated my case. he first asked me two questions: 1. have you moved since you filed. I said no. 2. Is your name clearly visible on the mailbox when the postman delivers mail. Said yes to this ( & put a label of my last name on my mail box after the call) Then he opened my case and after verification, he worked on it and after 5 minutes he told me that he has forwarded our applications and I should expect my card within 30 days. so lets see when they come.
so folks, call them asap and keep your fingers crossed.
Good Luck
Its funny that on the approval letter they say - call after 90 days if you have not recieved the card.
On their website they say - call after 60 days and
On the phone prompts they say - call after 30 days if you have not recieved the welcome notice.
. We waited 5 years to get this approval