Since the interesting stuff was actually in the U-Boot mailing list and other thread got a bit side tracked with irrelevant USB voltage related problems/observations - I am summing up to the best of my abilities here:
Status of the 8000 interrupts/sec problem
The 'dwc_otg' driver for the USB root hub (and the SMC NIC) in the Pi is doing 8000 idle interrupts per second. Tests have shown that disabling the driver and its 8000 interrupts per second can increase the processing performance of the Pi by as much as 20%.
This 'dwc_otg' driver is provided by Synopsis (not Broadcom). The source code for the driver is open but it is included in the Linux kernel tree - nor likely to become so, because it does not live up to established standards.
The current version of the driver included in Raspbian 2012/07/15 has a so-called 'SOF_reduction' setting which can reduce the interrupt load - but also results in some unfortunate side effects that impact responsiveness.
Stephen Warren (and possibly others) are working on a replacement driver for the USB root hub - but it is not yet fully functional nor available in Raspbian.
Please correct me as required