Will @mac.com e-mail addresses continue to work after the MobileMe to iCloud transition?
Yes.

You will have to opt in to switch your account over, but it sounds like it will be fairly painless as long as you agree to all the new (unannounced as of yet) license terms. From the Apple FAQ on the MobileMe/iCloud transition:


  Will I be able to keep my MobileMe email address when I move to iCloud?
  
  Yes. If you have an active MobileMe account when you sign up for iCloud, you’ll be able to keep your me.com or mac.com email address and move your MobileMe mail, contacts, and calendars, as well as your bookmarks, to the new service. When iCloud becomes available this fall, Apple will provide details and instructions on how to make the move.


If you do not opt in, you should expect your @mac.com and @me.com email to only work through June 30, 2012.