Is the 5870 the one you sweep a finger over the front of to turn it on? A friend has got one and it keeps getting turned on while in his bag.
I still haven't found an Ipad compatible bluetooth GPS which has an external antenna socket and an external power socket. I know one can bodge both of those; the former with a piece of foil stuck to the unit with a cable going to a remote passive antenna (which is how car handsfree kits for smartphones do it) and the latter by wiring up direct to the battery terminals.
It will be interesting if this is also a normal NMEA unit which works with non-Apple products (i.e. with 99% of the GPSs out there
).