I think there are higher priorities for improvements than a calculator.
How about video support, a2dp (or bluetooth stack in general), To-do and notes syncing (over the air with .mac, even .mac support), cut and paste, better caching in safari, local files and access, better handlng of on the go playlists in the iPod app, full keychain access to autofill websites (crucial for website reg of wifi sites) etc etc
I appreciate that not all that's gonna come right now but surely one of them could come ahead of a tweaked calc app. (All the above is possible on the current hardware btw).