I too like the idea of a calculator in Spotlight.
I think they could take it one step further, and maybe have other things you can do there just by typing in some letters. For example, if you want to copy a file from one place to another, rather than find it and click it and stuff you should be able to just say "cp" (for copy) "/path/to/file /path/to/new/file", etc. You could have a whole set of prefixes like cp, like mv (move), ls (list, you know, to see what's in a directory), and stuff like that.
Also it'd be kind of useful to have a record of the things you've typed, and what the results were, so maybe they could allow you to open a window, where you enter your "command" on a line at the bottom, let's call it a "command line" because that's what it'd be for, and then the results would scroll up above that line (together with what you typed. So you can see what it was you entered.)
That'd be cool.