I can confirm that I have seen my burns get worse after a firmware upgrade, and then improve over 5-10 burns using the same DVD stock and burning conditions.
I first noticed this on my Pioneeer 107D, but it may have been happening on the Lite-On 411S too, though I can't confirm this for sure, YET! I didn't realise up until about a week ago, that it was happening for sure, but as soon as I get some more DVD stock, I'm certainly going to be doing some testing to find out if the Lite-On is having a similar reaction to firmware upgrades.
As for what's causing it...
I've heard that laser re-calibration may be the culprit, but as far as I know at this point, it's all speculation, and no-one has given me a definate and reliable response about it yet, not on any forum.