Personally, I found the text for regeneration in the preview of the Beastiary interesting. For the round after the creature received damage by the correct energy type, any damage will get through the regeneration. It is literally turned off like a switch (emphasis mine):
[sblock="Regeneration"]Regeneration (Ex) ... Creatures with regeneration heal damage
at a fixed rate, as with fast healing, but they cannot die as
long as their regeneration is still functioning (although
creatures with regeneration still fall unconscious when
their hit points are below 0). Certain attack forms,
typically fire and acid, cause a creature’s regeneration
to stop functioning on the round following the attack.
During this round, the creature does not heal any damage
and can die normally. The creature’s descriptive text
describes the types of damage that cause the regeneration
to cease functioning....[/sblock]
I can see this causing some changes in how PCs fight some creatures. Now, after the caster burns the beasties, the fighter and rogue can get in some attacks past the regen - so long as they are near enough and quick enough to get in a hit. I can see this encouraging more team tactics and discouraging (or at least not encouraging) the need for a magical menagerie of weapons (flaming, etc) to deal with such creatures. Could be interesting, although it will take a bit to get used to.