From Buy.com's website:

Purchase a Slingbox at Buy.com and receive SlingPlayer Mobile software FREE! (Register your Slingbox and receive a free registration key from Sling Media) Offer valid from 8/21/2006 to 9/16/2006

This says you must register by 9/16.  Not very clearly but hard to argue...

Slinging Tivo Series 2, Tivo HD, and TW cable from a Pro. Slinging Tivo HD from a Solo, Slinging Humax DRT800, and TW cable from a Classic to Sprint Mogul, AT&T SX66, N62, 8525, and Dell Inspiron 8600 with Sprint EVDO over Windstream DSL.

I still haven't registered mine...  Because I bought mine back in July, and Sling's web site said the SB had to be purchased between Aug and Sept..  I feel like I'm being punished for purchasing mine early.

I don't understand why Sling couldn't just make the offer to everyone who didn't register?  Isn't the whole point of using the incentive of giving free Mobile software so people will regsiter, therefore Sling can keep track of the sales of the SB itself?