The corresponding server was equipped with a QMH2462 (HP branded QLA2462) running Ubuntu 8.04.02 Server and 2.6.24.53-24-server kernel and it didn't detect all devices it should. Two paths weren't detected. Because I'm not into FibreChannel stuff, further investigation by the SAN vendor showed that it was a login issue with an older but supported FC-Switch, while the paths over a newer switch didn't have this problem.
Unfortunately no solution was available by the SAN Vendor. Then I installed CentOS 5.2 and everything worked like expected, so the physics and configuration should be ok.

Some research on the internet showed that thist problem wasn't Ubuntu but Kernel-2.6.24 related
Finally I found this patch [PATCH 09/10] qla2xxx: Correct regression in relogin code., applied it, rebuilt the kernel & voila

All paths were available like they should, but...
...disk performance was really poor.

Messing around with different kernel versions from 2.6.18 up to 2.6.31 using in-kernel drivers, using drivers provided by QLogic and different firmware versions was unsuccessful. If the performance was ok some paths failed again as soon as data was read from or written to the LUN. (Building a kernel the debian-way is really pain if you are used to the gentoo-way i.e. and don't expect it to boot correctly without it's initrd)

Now I tried the latest kernel available in the ubuntu repos for 8.04 which is 2.6.24.60-24-server. Here the issue was that only one of three LUNs was detected, but this one was stable an showed good read/write performance.

Then I had a look at the module options again an finally the following was the missing magic to get the thing running

# nano /etc/modprobe.d/qla2xxx
qla2xxx ql2xplogiabsentdevice=1

update-initramfs -c -k 2.6.24-24-server
reboot