With version 9 physical constants are included in the system with units as a "Quantity". For instance, to input Planck's constant you type:
Quantity["PlanckConstant"]
This form can be discovered using an inline Wolfram Alpha query. To get this you type
ctrl+= and then input some free-form version of your query.
Note that I misspelled "Planck" and the Alpha query still worked. Alpha is pretty good at determining your intention.
I would prefer a palette for inputting these constants as well as for units. I have written such a palette for units and described it in a recent post
here, with a download link for that palette
here.
