It satisfies me to think that Leia remained Princess Leia Organa and never took a military rank. I do not think that this disqualifies her from being part of the leadership of Echo Base, as she clearly is. Look at the way General Rieekan relates to her.
I think it fits with her character to go and do a job which needs doing. As Scruffy has indicated, she would have been unable to continue the work she had been doing due to the Empire knowing she is part of the Alliance, the destruction of her homeworld and the impossibility of her continuing to use her role as a Senator for whatever purpose.
I don't see her as the sort of person who would boast to the Allaince of being "a career politician" or as someone who would see service of the cause as "subordinating herself to the military establishment". She strikes me as a much more practical and down-to-earth person than that.
I think her rank in the Rebellion, achieved through the years preceding ANH, would not have been lost due to the events in ANH. It would mean that she couldn't continue to do the same things that she had been doing, but it wouldn't mean that the Alliance would throw her aside, unless she was of no use to them. The fact that they didn't and she went on to be part of the leadership of the hidden Rebel base is testiment, I feel, to the fact that she was still of use as a leader in the Rebel Alliance.
I further feel that it is a bit of a guess to say that all the highest ranking people were with the fleet. We don't even know that the fleet had been assembled by the start of ESB. If my conclusion that Echo Base was the one and only base of operations that the Alliance had is correct, I think it follows that the leadership of that base would be key members of the Alliance.
I think that it works if we consider Leia as a member of the leadership of the Rebel Alliance, perhaps taking her father's place after his death.
Oooh... a thought for the 'What did the PT need?' thread...