Time for an update on my project 
lately I have been focusing on the wireless interface and power consumption of the sensor nodes. The ina333 op-amp workes great in my low voltage setup.
I have a prototype up and running on a breadboard(see the schematic)
To optimize the battery life, I am running a barebone AtMega328P on the internal 1Mhz clock, the bod level is set to 1.8v and is software-disabled while the processor is sleeping.
Also i am using a pnp-transistor to shut off the current to the loadcell and op-amp, so they only consume current a few milliseconds while sampling.
I am also using the Collector voltage of the transistor as my analog reference, so the voltagedrop of the transistor won't affect my measurements.
Since The Internal ocilator is'nt accurate enough, I connected a 32.768 khz crystal to the chip's clock-pins. This crystal feeds Timer2 and creates an accurate interupt every second. This way the the AtMega also works as an RTC, and I save the cost and powerconsuption of an external RTC.
All in all including the radio tranciver my crappy multimeter shows a current consumption of 2-5µA 
when everything except the 32khz crystal and timer2 is sleeping.
The wireless comunication was a real twist. I want the sensor nodes to sleep as much as possible, and there will be a lot of sensornodes trying to communicate with one "master-module".
The solution I foud, is to let the master-module decide when the sensor-node can start transmitting. So when a sensor-node is waking up from sleep, the master-module will already had started listening to that exactly node. Then when the sensor-node has sent it's data to the master-module, the master will respond by sending how long time to next datatransfer. The whole prosses of waking up the prosessor, wait for the op-amp to stabilize, take ten samples and calculate the average, sending the data, reciving the time to next datatransfer, and go back to sleep only takes about 100ms with an average current consumption of 6mA
If i take one measurment each hour, I would get an average current consumption of 5.25 µA 8)
Anyone who know how long this thing will run on a cuple of AA? 
the next tasks on my todo list:
- Use the internal bandgap reference of the AtMega chip to measure the battery voltage, and make a low battery warning.
- upload the measurements to Cosm.com using a gprs-module. (I just smoked my gprs-shield so this can take some time :))
- put everything in a watertight box and do a real longtime outdoor test of the loadcell.
ps. It looks like this thread is turning into a worklog. Mabye a moderator can move this thread into a more suitable category.