Yes!
It says the following on the certificate of application you received:

"Such persons may engage in employment without a work permit and whilst their application for a Residence Card or Permanent Residence Card is under consideration."

 

We received it after only a few days. As far as I remember, European Law says that they are supposed to give it to you "immediately".
Ours did not have a reference number - we later found out that they use the recorded or special delivery number to find you in the system. So it's a good idea to keep that receipt you got at the post office even after you get the certificate. ;-)