Although the Dell Streak is designed for use in landscape mode (as evidenced by the orientation of the touch button icons), the volume buttons actually prioritize portrait mode.
When you hold the Streak in landscape mode, the right side of the rocker decreases the volume and the left side increases it. Turn the device 90 degrees to the right and of course everything is fine: up to increase, down to decrease.
Unless there are two sets of volume controls on a device, one orientation always has to "lose" in this situation. It's just strange that the Streak's primary orientation is the one to take the fall.
Update: This seems to only affect the leaked Android 2.1 O2 ROM.