Aside from ludicrous Apple rumours involving unicorn blood and Steve Jobs' sweat, I rate the iPad 3 in September rumour pretty high in the stupidity stakes. All seemed to stem from a single off the cuff comment too.
From a production and sales point of view it makes zero sense, and the idea that super high res screens - which exist, but are barely out of the prototype stage, never mind mass production - are coming in 2 months is ridiculous.
At an absolute push, it's not inconcievable that higher capacity models may appear, but I find even that minor difference unlikely too.
Of course, if it did appear in September.....egg on face time, heyho!
Higher res screen is inevitable on the 3 eventually though, it'll be it's main selling point, possibly with 4G I suppose.