i think you can fly til about 34-36 weeks... but it would be pretty uncomf for you then... it really depends on where you want to go as well.
I went to Qld at about 29 weeks and that was great, I handled the flight fine and was still able to be relatively active and enjoy the holiday a bit more than if I had gone a few weeks later...
As you get bigger the hotter weather is less comf as well and in preg you are more sensitive to the sun. I am usually a major sun fan but I find now around 25 degs is lovely and you can still go to beach or lie by a pool...
enjoy!!! i loved our holiday and found it so good to relax with DH. I didnt want to leave!