When I plug in my usb floppy drive, dmesg shows this:
As you see, no device (such as sd0) was associate with the drive. What can I do?

There's nothing further in the dmesg output? Unfortunately that device would seem to not be supported (I have a digital camera that yields similar results). You could try sending a message to misc@ to see if anyone has a patch that would enable it, or to see if someone has a good work-around. If you don't know what misc@ means, then look at www.OpenBSD.org under the mailing list section.

Oh by the way, what version of OpenBSD are you using? There have been a significant amount of USB devices added from 3.4 to 3.6-beta. You could try a snapshot of 3.6-beta and see if that device is now supported.

thanks for the reply and suggestions, chort. to my (untrained) eye, the rest of dmesg isn't relevant. i did a search of misc before posting. i found a recent post
describing the same problem but no solution. i'm using 3.5, by the way.