Just one thing, what is the intention for this data? Purely datalogging? If so, rob is right. There is a non official firmware (if any of them actually ARE official lol) to do this which you will have to be mailed as I think it isn't public domain.
As for the circuit, a generic LM1815 isn't difficult to construct using a DIP part and a handful of discrete parts on some breadboard. There are examples in the datasheet, but I am not saying to much more as I would only be giving bad info.
There is a lot of info about on this somewhere. Either here or the dreaded wiki. I remember reading about it.
Good luck with whatever you end up doing!
-Gavin
*EDIT* See what happens when I type about stuff I have no idea about. The firmware is in the software forum, samabas has it in the 1.1.47 experimental link, and looks like MWFire has it working...... I'm gonna top up on coffee! Sorry for the mis information! */EDIT*