UPDATE: Sending from one of multiple accounts appears to be possible.  Check out this screenie of the Gmail import process into Google Apps:

Scott Hanselman writes:

I have two domains, one for each family’s last name. I could certainly create a separate Google Apps account for each last name, and you’re welcome to do that. However, I chose to put Hanselman.com as the primary domain, and “Smith” as the Domain Alias. Everyone will be able to choose what last name they want their “From” to show, so it doesn’t really matter.

Once you’ve signed up your first (primary) Domain go into the Domain Settings|Domain Names control panel in Google Apps and create a Domain Alias. When you “claim” ownership for a Domain, Google will give you a big long number and say “go make a text file with this big long number inside, and name it this big long filename. Upload it to your website, then come back and tell me about it, so I can go check to prove you own this domain.” You’ll need to do this for as many domain aliases as you set up.

My question is: will I have the opportunity to pick which “outgoing” account I want to use? I can do this in Outlook 2007 right now, and it’s useful. Sometimes it’s necessary, as when I want to send corporate e-mail for company X using my anthonys@companyx.com domain, and e-mail for company Y using my anthonys@companyy.com domain.

Will report back after I’ve found the answer.