Tapping Arduino into Rovio power supply

Hey, long time lurker; first time poster. I've been playing with Arduino (first Duemilanove, then Mini Pro) for about 9 months now, slowly teaching myself about circuit basics & microelectronics. I've always wanted to learn about electronic prototyping, but never managed to make time for it (I'm a applied research mathematician). The arduino platform and this forum have been godsends in teaching me the basics!

Recently I've been experimenting with augmentation of the Wowwee Rovio (a roving webcam robot that operates over wifi). Using OpenCV 2.1, I've created a heads-up-display of the Rovio's webcam for my netbook. This lets me use all of the features of OpenCV (detection algorithms, motion & color trackers, Kalman filters, etc) to make a genuinely functional HUD. With an arduino mini pro, I also connected a Wii Nunchuk to the netbook over bluetooth. Then using Python wrappers for the Rovio firmware, I can control the Rovio using the Wii Nunchuk. I also created a script to turn the Rovio to follow a moving target (tracked using OpenCV). Not perfect, but functional & a ton of fun!

Now, I'm interested in physically augmenting the Rovio with actual sensors (rangefinder, gyro/accelerometer, etc... haven't yet decided). To this end, I'd like to run an arduino (either 3.3v or 5v) off the Rovio's power supply. The Rovio is powered by a 6V NiMH battery pack (3000mAh - I'd like to upgrade that some time in the future too) using a deans ultra connector. Once powered, the arduino would transmit sensor data back to the netbook directly (rather than trying to interface with the Rovio's wifi), likely using a BlueSMiRF Gold (or maybe Xbee; you get the idea).

So I came to ask a pretty simple question I guess: what would be the prefered way to 'safely' tap an arduino into this power supply? Should I just 'split' the power and ground wires from the battery pack, with one branch going to the arduino & other to the Rovio's circuitry? Should I regulate the voltage/current limit/etc? and how would the situation be different between, say, 5v Duemilanove and the 3.3v Mini Pro?

I'm also planning on disconnecting a series of 6 very large, very bright LEDs on the surface of the Rovio (which serve no functional purpose). I'm thinking that this would free up enough amps during normal operation to balance the additional load from the arduino+sensors+transmittor.

A million thanks in advance!

Once powered, the arduino would transmit sensor data back to the netbook directly (rather than trying to interface with the Rovio's wifi), likely using a BlueSMiRF Gold (or maybe Xbee; you get the idea).

The ratio of success/failure on the forum for XBee far exceeds the ratio of success/failure for blue-anything. I'd recommend using the XBees.