Read this
http://www.uscis.gov/graphics/publicaffairs/statements/receipt_delay_42706.pdf.
"Due to an unusually large volume of receipts at the Service Centers during the first week of April, it is taking longer than expected for USCIS to enter cases into the system. Depending on the type of case and service requested, it may take nearly two weeks to generate and mail receipt notices. Once the receipt notice is generated, it may take an additional 2 to 3 days for customers to receive their receipt notices in the mail.
USCIS has taken measures to address this situation and expects to be current with its receipting by mid May."
Anyways, you can get your application number from the back of your check. Most of the banks have online scanned images of the cashed checks.