... well kind of!
Google has launched a calculator function (Google calculator) which also works very nicely from the Google search field built into Safari.
Fire up Safari and type an expression into the Google search field. Hit enter and Google gives you the result. As well as simple arithmetic (eg 5+2*2), it can also do more complex stuff like sine(30 degrees) or e^(i pi)+1. It knows about physical constants and can also handle conversions for example, 100 miles in kilometers or 160 pounds * 4000 feet in Calories.
[robg adds: Obviously, you can do this from any application that accesses Google; it works really well with the pop-up search engine front-ends such as Searchling and Huevos.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=200308130718081