With version 9 physical constants are included in the system with units as a "Quantity".  For instance, to input Planck's constant you type: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. 

You can still use the <<PhysicalConstants` package in Mathematica 9 if you would like. 

There is no new PhysicalConstants package, but instead the Quantity framework, which handles all kinds of different units and constants.  See the documentation on Quantity for more information.