according to schematics it is a battery fuel gauge on i2c bus #2
also there is sense resistor to gazoo chip ( twl4030 )

in kernel i find this
"++CONFIG_W1_SLAVE_BQ27000=y
..........
++# CONFIG_BATTERY_BQ27x00 is not set
.........
++CONFIG_TWL4030_BCI_BATTERY=y

"

From datasheet ( tps65950 hard & soft compatible with twl4030 )
.....
The BCI also performs monitoring functions:
• ac charger detection
• VBUS detection
• Battery detection
• ac charger overvoltage detection
• VBUS overvoltage detection
• Battery overvoltage detection
• Battery voltage level detection
• Battery charge current level detection
• Battery temperature out-of-range detection
• Battery end-of-charge detection
• Battery overcurrent detection
• Watchdog

bq27x00 features

* HDQ (bq27000) or I2C (bq27200) Communication
* Reports Accurate Time-to-Empty With Measured Load and Historical Maximum and Standby Loads
* Reports Temperature, Voltage, and Current
* High Accuracy Charge and Discharge Current Integration with Automatic Offset Calibration
* Requires No User Calibration
* Programmable Input/Output Port
* Internal User EEPROM Configuration Memory
* Automatic Capacity Reduction With Age
* Stable Oscillator Without External Components
* Dynamic End-of-Discharge Detection Delay to Allow Use in a High-Dynamic Load Environment
* Automatic Sleep Mode When Communication Lines are Low
* Available in a Small 3 mm x 4 mm QFN Package
* Five Low-Power Operating Modes
o Active: < 90 µA
o Sleep: < 2.5 µA
o Ship: < 2 µA (bq27000 only)
o Hibernate: < 1.5 µA
o Data Retention: < 20 nA

my opinion :
- it is possible to find bq27200 chip on the board but nokia can choose to not put in future. this chip is not mandatory for n900 running but can do better monitoring . nokia put this on the board but don't have time to write the software, an appendix almost useless until somebody choose it.
- battery can be monitored by software through twl chip
- it is a wrong initialization of driver for bq27200 ( i2dc device 0x55) or driver missing entirely even if it is activated by "CONFIG_W1_SLAVE_BQ27000=y" . bq27x00 chip is made in two bus variants bq27200 for i2c bus and bq27000 for hdq 1wire bus
so something is wrong in software, schematics make sense.

bq27200 chip is near blamed isp1707 usb transceiver chip