That was the inspiration for this site a few years back. I wasn't a Mac user at the time so couldn't even try Soulver myself but thought it was a cool idea.
Soulver's WolframAlpha support is great, too. No need to switch to a browser to bring in some real world data.
How do you do this?
Settings > Calculator > Wolfram|Alpha. You provide a Wolfram ID from WolframAlpha and then "= ?" in a sheet will query WolframAlpha for the previous variable.