Hiya!

I followed the instructions on http://rpi.fatdog.eu/
However, I've made a modification by creating an LVM instead of normal partitions.
Everything installed fine, but I could not add the important information about initrd to LILO, because there is no LILO and of course I ended up with an unbootable Slackware installation.

How should I proceed on the next attempt?

Amongst the other options you need to add "-L" so that lvm is added to your initrd, and you might also need to add the necessary modules via "-m" depending on the kernel you are using.
But I don't see much pint in doing that on the Pi, unless for some reason you are also doing luks encryption.

Can't give you any more detailed info as I don't have a Pi now and I can't remember the details dating back to when I had one lent to me for a short time.

Ok I checked at work and I still have the SD card on which I had Slackware running on the Pi.
Unfortunately at the time I tried none of the community images booted on the device I had lent to me so I did it according to what I documented as emergency way of getting slackware on the Pi here below the community links.

Anyway on the boot partition in my SD did not have an initrd, not sure if fatdog's image has:
if it does you will not need to edit config.txt,
if it does not you should add something like this to config.txt "initramfs initrd.gz 0x00800000"

You will then haveto create an initrd image. If you already have initrd you will need to figure out what options it was built with and add the lvm modules via -m and enable lvm root via -L, if fatdog's image does not use initrd you will only need to create initrd with "-m dm-mod -L" options but do checkout if dm-mod has dependencies as on my kernel build it's built into the kernel.

Hope this helps more the the previous my post.

Aye, this explanation seems like it should work. I won't have time to play with it during the week though. I'll report back in a few days.

FatDog.eu images uses initrd for the slackware arm installer which is deleted towards the end of the install process.

Unfortunately I was not able to create an initial ramdisk (it was spewing errors about files it couldn't find), so I couldn't follow your advice. I wasn't in the mood of tracking down the issue, so I just followed the tutorial and installed Slackware on normal partitions, sorry :P

Well unless you wanted a fully luks encripted syeytem it does not make much sense to run LVM on a single SD card.
Were you looking for help on a LVM installation ? if so I guess most of what I said is inapplicable ... but if you do an ordinary install you might be able to use the SD you installed on to make another SD with just the fat partitionb and the rest in LVM by using another computer.

As of yesterday evening I now have a Pi too ;-)

Nah, no encryption. I only wanted to have a more flexible patrition scheme. I've set my /var/log partition to be 200M, and I wanted to be able to easily shrink /home and expand /var/log if needed. I guess, I'll just use gparted if that time comes

For personal use and up to 60Gb or so, I don't have much of a problem in doing a 3 partition install (boot, root and swap).
In the past (before LVM support in linux) I used to spend endless hours trying to decide how much space I'd give to root, home, var, usr ... etc. I allays seemed to end up in a situation where I needed a certain amount of space and I actually had it but spread across several filesystem making it unusaqble directly. I then decided that for my personal use I could do with just the partitions strictly necessary.
Ok technically boot partition is not mandatory but 30 odd Mb aint gonna hurt anybody.

Back to making your own initr ... what was mkinitrd looking for ?
Is your system capable of creating PV, VG and logical vomume ?
If you can create a filesystem on a LV and the mount it and use it I suppose all you need are the bits and pieces necessary for the initrd creation.