I have installed OpenBSD 4.6/i386 on the Dell Vostro A90 / Mini 9. The install is absolutely vanilla, no kernel recompiles or anything else. I had to replace the WiFi card with an older Intel 3945abg, but beyond that, no changes were made.
The following hardware is tested and working:
Keyboard (X and console)
Mouse (X, haven't tried console mouse)
SD card reader - also works with SDHC
Bluetooth is recognized but I haven't tested it. Same for the audio and webcam.
But out of the box, a lot of stuff works! So for anyone considering the Mini / Vostro as a target for on the move OpenBSD, jump in, the water's fine.