The ST-made accelerometer itself most likely draws the 400 µA as specified in the datasheet from its 2,5-3,3 V supply as it's most likely set to the lowest refresh rate of 100 Hz, so the sensor draw is certainly not "a lot more". Not sure how the drivers handle it, but reading three 8-bit values over I2C, possibly comparing them to a few previous readings and then placing them to the specified location should not be "a lot more" power consumption either, even when done at 400 Hz (the maximum for this model). The accelerometer even allows setting limits within the control registers and just sitting idly while the accelerometer does all the job and generates an interrupt when the limits are exceeded (e.g. close enough to ±1 g to tell if the device has been turned). Plus it's most likely turned off like the ambient light sensor is when the screen is locked and the device in general goes to powersave mode...

The increased battery draw is more likely connected to the animation requirements for orientation-aware software; it's one more (and possibly frequently-used) animation to push through the GPU and possibly even doing some CPU work to rearrange and format the items being viewed and remapping the coordinates. Just flipping the view without any animations is painfully slow even on desktop systems (anyone who has tried to use the pivot capabilities of most flat panels computer displays are most likely familiar with the flickering and messed up desktop elements during the surprisingly long time it takes for the system to realise that everything is standing on its side now), so its drawing "a lot more" power even in systems that are designed to be orientation-sensitive is hardly a surprise. The effect can be cool, yes, but it does use some power - there's no such thing as a free lunch. I have currently all turning interactions on as I will charge the device every night anyway - the automatic rotation must be done really badly in the new Qt for it to actually reduce battery life by a non-insignificant amount. I wouldn't worry too much.