I was reading yet another thread over at the Piano World forums on composing music away from the piano. Most can't do this. Not because they actually "can't" but because they find it easier to notate ideas in front of the keyboard.
I actually have a happy medium to this "problem." What I do is jot down the first 2-bars of the musical idea. This is usually a melody so it's quite easy to notate. For example, it might be something like this in 4/4 time: Measure 1 = half note - quarter note - quarter note. Measure 2 = quarter note - quarter note - quarter note - 2 eighth notes.
I know this looks weird written out like this. Nevertheless, this is a great method for capturing the main rhythmic idea. Then, when you get to your piano, you take this rhythm pattern and assign note values. Now you have the beginning of a theme!