I'm not throwing out any numbers here and percentage increases are maybe not the best gauge as it's been pointed out that 360 sales have been at historic lows prior to E3.
Also valid is the point that the 360 250GB is the same price as the Elite it's replacing, meaning the majority of the buyers will be current owners of the 360.
Of course a sale is a sale for the purpose of hardware tracking regardless of whether it's going to a first time buyer, or That Guy who claims to be on his 11th Xbox 360 and/or has an Xbox in every room of the house.
New buyers will be jumping on the inventory clearing sales of the previous models though. $150 for the Spring Arcade bundle is a casual purchase, and many of these buyers are probably interested in buying Kinect upon release.
So the inventory levels on $149 Arcades will probably have a fair effect on the hardware numbers until levels start to thin out.
Not so sure about the 120GB Elite at $249, as it only represents a $50 savings over the new model with a larger HDD and more importantly, built in WiFi for those who need it.
I'm guessing that inventory levels on both SKUs can't be extraordinarily high if MS planned the launch of the 250GB SKU properly, but we'll see how long they last.