I found that some other people also had problems (e.g. here), but I checked the man pages and everything in my rules seemed OK. Finally, I tried the following command, and everything fell into place:
sudo udevadm trigger --verbose --subsystem-match=dvb
In previous releases, the udev rules were evaluated as the computer starts. My guess is that as part of the boot-time improvements in Lucid, it now remembers the hardware configuration from one boot to the next and does not fire 'add' rules unless hardware has genuinely been added since the last boot. So, my solution is to explicitly fire the 'add' rule through the 'udevadm trigger' command to force the rules to be re-evaluated.