Sounds to me that Stephan's idea is best - being able to hide the buttons if you don't want them and being able to show them for beginners. Then we can both be happy. :)
Add my vote to the interface that has buttons which can be hidden or shown (and are shown by default.) In fact we could take a cue from the Windows calculator and just have a simple and then more advanced mode. In the case of the Windows calc it is Standard and Scientific modes, but Simple and Advanced might be a more BeOS-like approach.
I would have to play around with DeskCalc and other BeOS calculator apps before I could come up with a good design to differentiate between what is simple and advanced, but I doubt it would be that complicated.