In 4.0 my USB cable worked as USB host without a problem, but when I installed a March snapshot it stopped to work. After a few hours of confusion and searches I finally cut the cable's connector and discovered, that my USB cable was not actually a Z-compatible USB-host cable (the two contacts were not soldered together). I soldered them together and after that it worked as before.
I did not check the unsoldered cable with Linux, but I think it wouldn't work. Looks like the older OpenBSD releases did not care about the connector, and probably the USB-handling changes have something to do with this.