It is possible. Garageband's musical notation abilities are a bit limited though (in GB2 you can't even print out the notations, not sure about the GB3 as I don't have it yet), it also isn't as accurate or editable as other apps made more for that specific purpose such as Harmony Assistant or Easybeat, or even full featured apps such as Logic or Cubase.
On to it though, I don't know which version of Garageband you have but in Garageband v2, open up the Track Editor (command+E) then when you have your piano track you want selected look at the bottom of the track editor and there should be a bar/note selector at the bottom of the 'Region' section. Hit the note selector and it should display musical notation instead of the bars. Small screenshot attached...