A rule of thumb about current lithium batteries is that they can be safely charged/discharged at a 1C rate, meaning that a battery can be charged or discharged at it's rated capacity. Say you are using the standard 6700 battery that is rated at 1350 mAh @ ~3.7 volts. This means you *should* be able to charge the battery at a maximum of 1.35 Amps per hour or possibly discharge the battery at 1.35 Amps for 1 hour. However, most of these batteries will have some sort of "smarts" to them and may shut itself off at a current draw less than it's 1C rating, would depend on the application.
I don't exactly what you were thinking of "tapping" the battery for, but if it is an external GPS to be integrated into the back of the 6700, then someone is working on it already:
http://ppc6700users.com/forums/viewtopic.php?t=4494
*And yes there are lithium polymer batteries that can safely discharge at rates of 15C or so, but they aren't too common, especially in the cellphone market (if at all or ever). The charge rating is still roughly at 1C though.
**Just a random link if someone wanted to "play" around with other battery options.
http://www.batteryspace.com/index.as...S&Category=895
Although I'd recommend a lot of reading on these for any sort of DIY projects.