Hi,
not sure if this is the right place to post this but I have published a design for a Data-logger that I have been developing http://interestingto.me.uk/Open%20Source/low-cost-low-power-datalogger if anyone is interested.
It is based on a Atmega 328 so is Arduino programmable if you have a programmer such as the UsbTinyISP.
Looks very cool. You think you can fit the logger in an attiny85 or 45? How long does that battery lasts?
For my application the Hex file 58kB so that wouldn't fit into an 8k ATTiny but I am using all the available libraries (SDFat, DS1305, TWI, BMP085) so you could probably get this down by reducing the size of those libraries. Whether you would get it down to 8k though, I don't know.
As for power, it depends on your battery, the on-board battery just runs the Clock, the processor needs a separate battery. When sleeping, at the moment it is pulling about 400uA pretty much all of which is the DS1305. I am going to look into running the DS1305 from an ATMega pin so that I can force it to run from its back-up battery in which case it is in the nA range. Even at 400uA most of the time, a 4.5AH battery will run it many, many thousands of hours