A Video iPod would seem logical since its been a year since the 3G iPods were announced, and M$ is ahead of Apple with portable video. BUT, this is a broadcasters convention. What do they care about that? Nothing.
Some say it will be a TiVo like device. Again, this is a broadcaster's convention, and generally, broadcasters HATE TiVo devices because everyone scans over the ads causing revenue to go down.
No, I hate to be the spoil sport, but I'm guessing whatever Apple is announcing will be boring to those not in the broadcasting medium. In fact, I'd guess it will be either a FCP upgrade or maybe a Shake update or price decrease. Whatever it is, remember, Apple's presence at the NABfest is to convince BROADCASTERS to use Apple software.