Finally something positive happened: my Receipt Number all of a sudden started working when I check Case Status online, and message says:
On April 7, 2011, we received this N400 APPLICATION FOR NATURALIZATION, and mailed you a notice describing how we will process your case...
Looks like my Case now have 2 NOAs: one on January 28th and new one on April 7th.
When did your check get cashed?