I started June 7 and everyone in my class got their first choice. Some were even able to change afterwards. The class behind us got mostly CLE and EWR.
They have system-wide bids every 6 months or so, which is when you can bid into another base. The reason our class got what we wanted is that a system-wide bid just came out or is coming out (can someone on the line verify this?) and they said it was easier to give everyone in our class what they wanted right away than send someone to a base only to have them transfer out (mostly to IAH) less than a month later.
So even if you get your second or third choice, you should be able to bid to your first choice in six months or so, although you may sacrifice base seniority (ie. ability to hold a line vs reserve). Of course, if you are in the last group to get hired, you may not have that choice.