I sent my RFE response (EB-2 NIW) by express mail on April 18, 2007. Signed for USCIS 11.45 AM on April 19, 2007 as per USPS delivery confirmation. LUD and CRIS email, received on 20.45 PM on April 19, 2007 (same day) saying "received rfe response and case processing has resumed"
That was pretty fast though, but waiting now for 11 days for a decision.
I think it should be OK, this is the H1B vias cap month, so they are freaked out I guess.
Goodluck