I've got a mesh with lots of faces which I'd like to solidify. After making it solid, I'd then like to simplify one side (via the decimate modifier) to reduce the polygon count while leaving the other side untouched. I thought one way of doing this would be to assign the new vertices generated by the solidify to a vertex group, then apply the decimate to that group. Is this possible?
You can use a workaround to select them all at once.
If the modifier isn't applied
Assign generated faces to new material and select by material
Create one more material for your object, you may leave its default settings as they aren't important. The only thing which may be changed is the name of the material so you are able to find it quickly (and you should keep in mind that this material must be used only for this purpose in order this trick to work).
Then, in the options of the Solidify Modifier find Material Index Offset which lets you assign a material to faces generated by modifier. Set it to the value of the material you just created.
From the wiki:
Material Index Offset
Choose a different material to use for the new geometry; this is applied as an offset from the original material of the face from which it was solidified.
After the modifier is applied, you are left only to select one of the faces generated by modifier, and click Shift+G > Material. All the vertices with the same material will be selected and you can add them to vertex group.
If the modifier is already applied
It should help selecting all the loops around rims and using Select > Select Loop-Inner Region: