Get the newest snapshot from the CVS repository under: http://at76c503a.berlios.de/cvs.html
Unpack the archive in /usr/src/ - assuming you have already compiled the kernel version 2.6.12.4-EpiaV (Adapted in the Makefile of the kernel-source directory)

tar xzvf at76c503a-cvsroot.tar.gz
mv at76c503a CVS
cvs -d `pwd`/CVS co at76c503a
cd at76c503a
make KERNEL_VERSION=2.6.12.4-EpiaV
make KERNEL_VERSION=2.6.12.4-EpiaV install
/sbin/depmod -ae -F System.map  2.6.12.4-EpiaV

Note that you don' t need to specify all kernel-related version stuff when already running the kernel you compiled the driver against.
I have found out that there are several Firmware Versions available for this WLan Module - and I still don't know where I got the latest version - but in case someone looks for it - check it out firmware version 1.103.2 #175.
Please note that I tested it with USB device ID: 03eb:7605 only.

Note that some people have reported version 1.103.2 not to work on their hardware. In that case - use firmware version 1.101.0 #84. This version seems to work almost everywhere.