I don't see anything like that here on OS X, using this build:
http://www.luxrender.net/forum/viewtopic.php?f=30&t=10223

Which exact Luxrender version are you testing? And just switching the render engine causes the CPU usage, not other steps needed?

Also tried this on Windows now, CPU usage is around 0-1% here. Also it might be interesting to check if it's the blender process that is using 70% of the cpu, or luxrender or some other process started by the addon.

It's Blender's process. LuxRender is v1.2.1. Also noticed that in dual core the other core goes to 100%. When you switch back to Cycles or BI the cpu usage is not released.

Updated video drivers to 320.49. Cpu usage is now 60%, but only for about 30 seconds, then it drops to 15-20%.

I tried Luxrender 1.2.1 (with and without OpenCL) and yafaray 1.5.0 now with Blender 2.68 RC, but can't see the issue still.

Do you have any other addons enabled that might be influencing this? I suggest to try File > Load Factory Settings (this is not permanent, just temporary as long as you don't save user settings), and then only enable the Luxrender addon, and select it as render engine to verify.

When I load factory settings Luxrender is already in the render engine list, but it's not activated in the addons list and I can't activate it. It gives error message with last line KeyError: 'Node categories list 'LUX_SHADER' already registered. However if I switch to Luxrender engine it still has 70% usage.

I guess we need someone else who can confirm this, maybe you could ask in the Luxrender forum?

Changed 3D settings to emphasize performance in nvidia control panel. Now it's more random, sometimes when I switch between Cycles, Yafaray and Lux I can get Lux or Yafaray work properly.

My laptop with GT240M has the same problem, but it's little bit different which I now noticed. In start up Luxrender fails to load pylux dll so it's using a regular one (whatever it is). When I switch to Luxrender it works as it should. But when I load a scene with Luxrender it goes to 40%.

I can ask in Lux forum. It may be that at least table top computer users don't even notice this.

My best guess is that there is something that luxrender is listening for updates and that there is something causing it to be called continuously. Maybe it's an addon, maybe it's a particular scene setup, ... it's probably easy to track down if I can redo it but for now I'm not sure what to do.

It could be something related to graphics drivers / cards, but I'm not sure if Yafaray actually uses the GPU, and I certainly wouldn't expect it to when just loading a file without actually starting the render.

If you check the log in the thread on the Lux forum, you can see Pylux was stripped out of the instal and the issue was still therel:
http://www.luxrender.net/forum/viewtopic.php?f=16&t=10247

That means the addon no longer has the Lux core and can't do actual renders. So any CPU use at that point would be due to API calls, etc, making Blender do things itself.

Both Yafaray and Lux start to use also GPU when turned on, here is the output of Process Explorer posted on Lux Forum (showing LuxRender's CPU and GPU usage):
http://koti.mbnet.fi/paulkp/temp/procexplorer.png

Hey, official 2.68 seems to have fixed this. I haven't seen any constant cpu usage after couple of tries.

Ah, good to hear. I don't know exactly what would have done it but will close as fixed anyway, can always reopen if it pops up again.