All times in this post are my (Pacific) time.
Feel free to copy/paste this entire post to other Firefly forums, but please credit http://www.JacquelinePassey.com/ and let people know they can go there for updates.
Here is a list of cities with links to buy tickets. The tickets will say they are to "Special Advance Preview" instead of Serenity. Many are currently showing as sold out, however, there are rumors about possibly upgrading some of the sold out screenings to larger rooms with more seats.
Atlanta, GA http://tinyurl.com/7dfdx (11:00 AM rumored sold out, someone called the theater)
Austin, TX http://tinyurl.com/87fqo (11:25 AM rumored sold out, someone called the theater)
Boston, MA http://tinyurl.com/9947j (11:43 AM rumored selling tickets again, possible theater upgrade?)
Chicago, IL http://tinyurl.com/ey9wo (11:26 AM rumored sold out, someone called the theater)
Dallas, TX http://tinyurl.com/ax5s5
Denver, CO http://tinyurl.com/8qytg
Kansas City, MO http://tinyurl.com/dwfqq
Miami, FL http://tinyurl.com/77t32
Norfolk, VA http://tinyurl.com/czddo
Philadelphia, PA http://tinyurl.com/7wk9c (11:41 AM rumored selling tickets again, possible theater upgrade?)
Phoenix, AZ http://tinyurl.com/dp63y
Providence, RI http://tinyurl.com/8zp5o (as of 11:30 AM Pacific / 2:30 PM Eastern rumored to still have tickets!!!)
San Francisco, CA http://tinyurl.com/a3r7y (11:15 AM rumored sold out, someone called the theater)
Seattle, WA http://tinyurl.com/ccjjz (11:00 AM rumored sold out, someone called the theater)
Washington DC http://tinyurl.com/d9pcl (10:48 AM rumored sold out, someone called the theater)
Other cities that will have screenings but I do not have links for yet:
Hartford, CT (11:43 AM call automated phone line to buy tickets 1 860 646 9800)
Las Vegas, NV (11:15 AM rumored not selling tickets until May 15, someone called the theater)
Portland, OR (11:00 AM rumored not selling tickets until May 18, someone called the theater)
Please post complete links to any other ticket sales pages for other cities you find in the comments.
The official site for this stuff is http://www.cantstopthesignal.com/ but it was not being updated most of the morning. As of 10:55 AM they have finally updated it with a list of theaters and contact info! As of 11:34 AM contact info has been removed from that site, the theaters were complaining about the deluge of calls. :)
I am monitoring Browncoats, FireflyFans.net, Livejournal, Serenity Spoilers, and Whedonesque. (At lot of these sites are crashing right now as fans scramble for tickets, though.) Please let me know if there are other forums I should be checking for updates on screenings.