I ended up with EHCI once on irq 10 and OHCI three times on irqs 5, 14 and 15.

I tested the 6 ports with my Linksys 64MB "memory disk", and each instance of OHCI handled 2 ports. Reading in the motherboard manual I find this rather interesting statement
Each of the six USB ports can be automatically routed to support a High-speed USB 2.0 devices or Low-speed USB 1.1 devices.
Sounds like EHCI only comes into play if there's a 2.0 device plugged in. I don't have any USB 2.0 devices to test it out with.

I got the memory disk for $10 at Fry's(it lists for $45). I suspect the 1.1 versions of the memory disks are being cleared out for faster 2.0 versions.