Ever since I found that Snow Leopard switched its file size calculations reported in the Finder from Base 2 to Base 10, I've been eagerly searching the Internet, trying to find a way to keep (or switch) between Base 10 and Base 2.
Download brkirch's switchDiskSizeBase from the author's website.
It's a command line program that works on OS X 10.6 (Snow Leopard) and 10.7 (Lion). From various posts I've read about the software, you may need to periodically download a newer version that will work in the event a Software Update breaks the functionality. However, the author appears to actively support it. The software is free and open source, but the author also accepts donations.
After downloading and unzipping the file you'll have a folder with the executable, and the source code and license. To run the program in Terminal, cd to that folder and type:
Enter your administrator password. You'll need to reboot to see the change. To switch back, just run the program a second time.
Here's a link to the discussion forum I found this hint from. I add the hint here because I have literally searched two years for a solution like this, but was only able to find this solution today. I'm sure more people will find the hint here on Mac OS X Hints than haphazardly finding the solution in an obscure forum like I did.
[crarko adds: I tested this on 10.7.2, and it works as described.]
Mac OS X Hints