In fact, even if I don't touch the brightness setting on the display, the instant I select the different whitepoint settings in the dropdown menu, the brightness changes tremendously (i.e., switching from native to 6500K results in an immediate dimming of the display).
This is perfectly normal. TFTs can not change their color temperature. They are fixed based upon the color of the CCFL backlight. In order to change the color temperature of a TFT you have to dim pixels. One thing a lot of people don't understand is that when your screen is displaying pure white, all pixels are actually OFF. When you're displaying "black" all pixels are on. The reason you pay attention to things like black levels are because the backlight actually bleeds through the pixels (which is why TFT panels have more difficulty achieving a good black level).
So, when you set your calibration software to "native" white point that means that all pixels on the display are off and that it doesn't adjust the color temperature. Instead it measures that white point and then starts correcting color. The resulting video LUT curve is pretty much going to be at a 45º angle going up from left to right. You'll see slight deviations of the R,G,B channels but that's only to correct the colors that are out of whack.
When you tell your calibration software to set the display to a specific color temp, it must do this through the LUT (lookup table) on a TFT. With a CRT what you used to do is adjust the individual output levels of the red, green, and blue guns to change the white point. Since we don't have individual red, green and blue guns* we have to start dimming pixels. This is equivalent to say creating a blank white image in Photoshop, creating a curves layer, switching to the blue channel and then setting the output to say... 200 (this will give "white" a yellow cast). As a result the overall luminance goes down. Your screen probably needs to pull two channels in order to get to the desired color temp that you are specifying resulting in an even greater reduction in luminance.
Some software is better at compensating for the reduction in luminance that occurs as a result of adjusting the white point via the video LUT but... every time you change your profile you do need to recalibration the backlight output in order to compensate for any loss in luminance as a result of profiling to a white point that is not the monitor's native white point.
* Now for the asterisk. Some displays such as the HP Dreamcolor, and Lacie 724 and 730 (which are actually the Samsung XR24 and XR30 respectively) have red, green, and blue LED backlights and with these screens you can actually truly change the white point of the screen without negatively impacting desnity resolution. These screens are however VERY expensive.
Cheers, Joe