Given that it's now almost October and Photokina 2012 has come and gone, I suspect the D5200 will be a PMA 2013 (Q1) announcement. Announcing it during the Holidays but not making it available will cannibalize current stock and clearance sales.
For what it's worth, the D3100 is on sale with an 18-55 VR for less than 500 bucks. That's a heck of a price for a very good camera, even though it's been supplanted by the D3200. Or look around - the more popular/reliable online retailers have refurbished models going for much less than that. I picked up my D3100 that way; I already have lenses and accessories - it was a steal at under 300 bucks.