No inside information, just a guess based on prior events. With Apple on the verge of breaking the 10 million download mark for the App Store, they will surely want to highlight this with the press and what better way to do it then at an event dedicated to a platform that supports the App Store. It wouldn't make sense for them to have a media event a couple months after the 10 millionth download as it will be relatively old news.
Also with iOS 4.3 with so many iPad 2 references, why would they put them in an update that would not be used with the iPad 2?
Then you have the BestBuy SKU slip. Does anyone know the time between last years slip and when they had the media event to announce the original iPad or was the iPad not sold at BestBuy for a few months after? My memory isn't doing me well right now lol