Certain USB devices not probed/attached on boot

[Dave Huang <khym%azeotrope.org@localhost>]

I've got a Soekris net5501, currently running 5.1_STABLE from Dec 19 
2010, although it's had this problem since I got it (running 5.0_STABLE 
from September 2009): it won't attach a 4-port USB-serial adapter if 
booted with the adapter plugged in, but if I plug it in after boot, it 
works fine:

ohci0 at pci0 dev 21 function 0: Advanced Micro Devices CS5536 OHCI USB 
Controller (rev. 0x02)
ohci0: interrupting at irq 15
ohci0: OHCI version 1.0, legacy support
usb0 at ohci0: USB revision 1.0
gcscehci0 at pci0 dev 21 function 1: Advanced Micro Devices CS5536 EHCI 
USB Controller (rev. 0x02)
gcscehci0: interrupting at irq 15
gcscehci0: EHCI version 1.0
gcscehci0: companion controller, 4 ports each: ohci0
usb1 at gcscehci0: USB revision 2.0
uhub0 at usb0: Advanced Micro OHCI root hub, class 9/0, rev 1.00/1.00, 
addr 1
uhub0: 4 ports with 4 removable, self powered
uhub1 at usb1: Advanced Micro Devices EHCI root hub, class 9/0, rev 
2.00/1.00, addr 1
uhub1: 4 ports with 4 removable, self powered
[ unplug/replug here ]
uhub2 at uhub0 port 1: ALCOR Generic USB Hub, class 9/0, rev 1.10/3.12, 
addr 2
uhub2: 4 ports with 4 removable, self powered
uftdi0 at uhub2 port 1
uftdi0: FTDI USB HS SERIAL CONVERTER, rev 1.10/4.00, addr 3
ucom0 at uftdi0 portno 1
uftdi1 at uhub2 port 2
uftdi1: FTDI USB HS SERIAL CONVERTER, rev 1.10/4.00, addr 4
ucom1 at uftdi1 portno 1
uftdi2 at uhub2 port 3
uftdi2: FTDI USB HS SERIAL CONVERTER, rev 1.10/4.00, addr 5
ucom2 at uftdi2 portno 1
uftdi3 at uhub2 port 4
uftdi3: FTDI USB <-> Serial, rev 1.10/4.00, addr 6
ucom3 at uftdi3 portno 1

I tried booting with a USB memory stick plugged in, and that was 
attached fine (albeit as ugen, since I apparently didn't compile in 
umass support), so the problem isn't that *nothing* USB gets attached on 
boot. On the other hand, a Logitech webcam also doesn't get attached on 
boot, but does if unplugged/replugged.

Does anyone have ideas about what's going on or something I can try?