It seems I am not alone:http://www.linuxquestions.org/questions/slackware-14/slack-11-acpi-and-udevsettle-problem-504032/http://ohioloco.ubuntuforums.org/showthread.php?p=5077994
There was another on these forums, but I can't find it now.
I had selected standard vga during the installation and had checked that vga=normal was in lilo.conf.
When udevsettle (Performing coldplugging) was running, the screen mode changed. In desperation, I changed lilo.conf to vga=771 (800x600x256), the screen mode doesn't change at all during boot and TV out works as it should. I also see the penguin now - cute.