I'm guessing Nintendo will not announce anything at PAX, but maybe it's bigger than I think.
I hate to say this, but what Nintendo needs is a launch on MTV. Show more games and more interesting stuff than the XBox 360 MTV launch, but otherwise, yeah, go for it. Whore some celebrities with no clue, talk about polygons and other stupid crap, but mostly, show off what makes the system unique. The point is to reach the casual crowd...while still delivering the goods that the hardcore crowd has been watiing for.
Of course the most likely scenario is that Nintendo will go to some already large gaming event, or will announce a big press conference, and then show us what's going on. That personally suits my tastes way more than a cheesy MTV launch, but I think that Microsoft had the right idea and the wrong execution.
Edit: summarized thoughts.