The i.Trek M5 works wonderfully with my n800. I also saw someone posted that it worked with n770 too. Opened Maemo Mapper and it automatically detect when Blue Tooth is on, on the GPS device.
As far as battery life, I haven't used it long enough to know.
I used it on a 4 hour drive, and wandered around on foot playing with it for a couple more hours (at least) and still had power.
It charges via mini USB port, so supplying power is easy.
My iBlue 737 is cheap, uses a nokia battery, and comes with both wall and car chargers.... all for $48.99 @ semsons.com. They throw in a free car pda mount. iIf you use the coupon code 'thanku', you get 5% off. Battery life is quoted @ 27 hours....and its pretty small.
The "generic" Palm bluetooth gps that ships with TomTom softare for the Palm works fine. as an added bonus the nokia is significantly more robust than the lifedrive, which has microdrive issues.
I recently bought on Ebay a Navman Spin 100 set-up for <£20. It came with Windows CE software on SD, but the BT GPS works fine on its own with MM. Battery life ~10hrs continuous from 3x AA batteries, or it has a 5v input socket...
