Thank you for posting the dmesg. You have an IDE hard drive (wd0), so I did not misunderstand the storage configuration.
As I mentioned above, both Softraid and USB mass storage will be assigned virtual sd(4) devices as they attach. In this particular dmesg, only a Softraid drive is attached as sd0, there are no USB storage attach / detach messages.
All but 5695 sectors of the underlying physical wd0 drive are allocated to the Softraid drive sd0, as you are using Full Disk Encryption.
Your USB controllers include ehci(4) for USB 2.0 devices, and ohci(4) for 1.1 and 1.0 devices. Your USB mouse is attached via the USB 1.0/1.1 controller ohci0, while your video camera is connected via the USB 2.0 controller ehci0.
I don't see anything problematic in this particular dmesg. Someone with more skills and knowledge might have a different opinion, of course.
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The kernel driver detaching the mouse on exit from X Windows that you have mentioned is a problem I am unfamiliar with. As it does not occur while using the mouse while you are running X, it is less likely to be a hardware problem with the mouse, and more likely to be a software problem of some kind.
You might consider reporting the problem to the Project. If you use the sendbug(1) script to structure a formal report, it will include usbdevs(8) output in addition to a dmesg which may be helpful to a developer.
http://www.openbsd.org/report.html