Should I use subdomain for google apps?

Hi all,
I’m working on a website that has 2 distinct parts:

  1. public information with a contact form, along with login fields to access admin area
  2. administrative area that will be powered by google apps for excel and file sharing.

I’ planning on putting the public site at domain and the admin part at admin.domain. I thought this might be better than having admin at domain/admin.

I’m having some trouble figuring out best way to do mail. I’d like gmail functionality, but dreamhost only allows a form to be sent to a dreamhost hosted email name.
I was going to set up dreamhost mail at domain level and have it forward to subdomain. I’d use the subdomain gmail address as main emain address.

Does this make sense? pros/cons?

Any help would be appreciated.