who should be included as guests for the rehearsal dinner?

I think it is generally left up to the wedding party, the families of the bride and the groom sometimes to include grandparents. Of course there are certain protocol when you are dealing with an out of town wedding where you have guests that travel further distances to enjoy your day but again that is really dependent on how elaborate you are going with your rehearsal dinner. Back yard BBQ's can be extended to out of town aunts/uncles but not at all necessary.

There are two ways to go...

The first is to invite just the people involved in the wedding:
Bridal Party and Dates
Parents of Bride & Groom
Grandparents of Bride & Groom
Any Readers or Musicians
Pastor

The second is to include all out of town guests. This theory is that it is your responsibility to entertain them for the entire weekend.