I just got wireless USB working on my SL5 laptop.  There was a lot to
figure out, so I am sharing it here.

I have a MyEssentials ME-1001-USB wireless adapter, which contains a
Zydas zd1211b wireless chipset ( bought from Fry's on saturday October
4 2008.  Note that chipsets change without warning!).  The device is
plug and play on an Ubuntu Heron 8.04 system (with kernel 2.6.24) but
not so with Scientific Linux 5 kernel 2.6.18-8.1.6.el5 . 

The SL5  zd1211rw.ko  driver is missing most of recent the USB device IDs.
So, it needs recompiling with some additions as shown here:

 http://linuxwireless.org/en/users/Drivers/zd1211rw/AddID

What follows is the recipe that works for Red Hat style RPM systems, and
something similar will probably work for SUSE.    I will let you Ubuntu
Debian Slackware Gentoo folks figure it out on your own.  CentOS documentation
refers to /usr/src/rpmbuild instead of /usr/src/redhat , but the latter is
where the files show up.

I found the source files for my kernel here:

 ftp://fr.rpmfind.net/linux/sourceforge/l/li/linux-ntfs/kernel-2.6.18-8.1.6.el5.src.rpm

Then the standard redhat style kernel preparation, as root:

 rpm -Uvh kernel-2.6.18-8.1.6.el5.src.rpm
 cd /usr/src/redhat/SPECS/
 rpmbuild -bp --target=i686 kernel-2.6.spec
 cd /usr/src/redhat/BUILD/kernel-2.6.18
 chmod a+w -R *            # this allows you to work as a normal user

# do these as a normal user

 cd /usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686/drivers/net/wireless/zd1211rw/
 cp -a zd_usb.c zd_usb.c.orig

# option one ----------------------------------------------------------------
# download   http://www.keithl.com/zd_usb.c  
# which contains the current device list as of 2008-Oct-05

# option two ---------------------------------------------------------------
# edit the zd_usb.c file yourself, copying lines from the most recent
# version here:

 http://wireless.kernel.org/download/compat-wireless-2.6/compat-wireless-2.6.tar.bz2

# Untar that into a handy place.  You will be needing the file   zd_usb.c
# to patch the similar file that comes with the kernel.  Otherwise, this
# driver source requires a much more recent kernel.
# 
# copy all the the DEVICE_ZD1211 and DEVICE_ZD1211B lines from the recent
# version   zd_usb.c   and patch them into the kernel source    zd_usb.c

# compile the module, as a normal user --------------------------------------
# using the recipe in   http://wiki.centos.org/HowTos/BuildingKernelModules
     
 cd /usr/src/redhat/BUILD/kernel-2.6.18/linux-2.6.18.i686/
 make clean
 make oldconfig
 make menuconfig     
 make prepare
 make modules_prepare
 make M=drivers/net/wireless/zd1211rw/

 su root
 cp drivers/net/wireless/zd1211rw/*ko /lib/modules/2.6.18-8.1.6.el5/extra/
 /sbin/depmod -a

# all done! ----------------------------------------------------------------

The Wireless USB should now be plug-and-play as eth1.  I did not have
to play with  /etc/sysconfig  or  /etc/modprobe.conf   
Note, this version of the driver, as well as the one running on Ubuntu
8.04 , does not provide signal strength to Network Manager.

The most recent version of this recipe can be found at:
    http://wiki.keithl.com/index.cgi?Zd1211WR

Keith

-- 
Keith Lofstrom          [log in to unmask]         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs