The number on the back of my check also came up in the system, after I changed the $ to a *. The result was "On September 14, 2007, we received...". The date that should be returned when you enter the real receipt number is the date your check was deposited for cashing. When my receipt number finally showed up online this morning, it displayed the correct date.
Ahh, that makes sense. I'll keep an eye out for NOA letter. Wonder why USCIS likes to confuse us so much. Sometimes I think they take joy in it.