I have the base filesystem (/) on LVM (reiserfs) because of ongoing problems with unsufficent space after distro upgrades. Only /boot is on separate ext2 partition because system will not boot otherwise (at least I read this - never tried).

After trying to download opensuse11, the download broke, the reboot-required sign in ubuntu 7.10 appeared and computer ceased restarting when following that advice.
The ubuntu splash-screen just appears but no progress is made and after some minutes I am on an ash prompt.

Starting rescuecd (I am new to this as well) revealed that there is 0 (zero) space on the LVgroup (I also read, that full LVgroups deactivate themselves).
Running vgchange -ay reports: VG activated !
This does not change anything however.
I believe that deleting the suse11.iso from within LV would solve my problem,
but how to achieve this?
Any ideas?

Does
vgscan --mknodes
help?

after many more hours of trying and reading I dropped systemrescueCD (www.sysresccd.org). It comes with LVM commands but lacks the opportunity to open the LV to acces the files.
I started the ubuntu 7.10 CD in rescue mode with bash prompt on /dev/volgrp/vol and suddenly I was able to access this file system.
This seems to be the key advantage of LVM, to have proper access to the files, even if a hardware defect blocks vital functions.
I found the very big file and deleted it, filespace consumption was back to as-usual, however even doing that booting of system was still not possible.

I decided, because files are perfectly readable, to save data on 16GB USB stick and make a fresh setup of ubuntu (this time 8.04) on a new Harddisk.

This is a bit time consuming, I just calculated data transfer will take in the range of 15hrs.

Whoa, you must have a lot of files or some very big ones or a very slow transfer rate!

in order to finish the story some new "developments". You are right, 15 hrs is a lot. So I went through the filelist and decided what really is worthwile to keep. I ended with about 5 GB data on about 5000 files. Copying on the USB stick took finall not yet 5 hrs.

The fresh setup I made on opensuse 11, just to give it a try since it looks a bit more useful than ubuntu and I have already 3 ubuntu 4.8 computers but only one with suse 10.3 which seem to be quite usefull.
In the course of the setup the HDD refused to start this was due a defective IDE cable.
Than half of my 1GB memory went off, I decided that 512 MB is good enough and finally after some operating time at irregular intervals the system freezes and only a restart brings it back to life. I presume motherboard damaged. This is the end....