What was the symbolence of the kiss?

I don't think it was supposed to go anyplace. Women sometimes form strong and almost erotic friendships, but don't necessarily act on eroticism.

Clarissa (by my interpretation) is a lesbian. She stifled her sexuality in order to Conform and abide by the rules of society. She is "the perfect hostess" and her bed in that attic-room of hers is a show of the sterile household where she lives with Richard Dalloway.
Peter interrupts Sally and Clarissa with, "Star-gazing?" - which in fact, they were. They were looking at a future which is far out of their reach.

Reading: Revolutionary Road

Watched: The House Bunny

I think that's the point. The movie compares youth and age: Youth as a time full of options, which is wonderful, but also forces you to make decisions on how you want to lead your life - knowing full well that a decision for Plan A might obstruct your way into Plan B forever. Age as a time when you have settled into this life, but go back to consider if you made the right choices. Young Clarissa had the passionate admirer, the sensible man, the radical friend to choose from, and her decision led her to where she is now.

I'd always be careful with interpretations like "she's a lesbian, ergo repressed, ergo unhappy" - life's never as simple as that, and you can always find good arguments for all decisions you could have made.
Moreover, Mrs. Dalloway may be reflective, but she doesn't strike me as unhappy - after all, she loves giving parties, and is in a position to invite even the Prime Minister. She has a daughter to be proud of, and her husband seems very caring.

I think it represents a lost possibility...neither good or bad. But I don't remember how it is addressed in the book.

The author Virginia Woolf was lesbian, however (in a platonic marriage to a man), so add that as you will. One of her nervous breakdowns was brought on by the one and only time they had sex together.

Sally was one option Clarissa had as far as love and lifestyle, as was Peter. Those two were passionate, daring young people who wanted to change the world and make a real difference. I'd say that choosing Sally would have been a stronger message about women's independence, as well as living a life of passion.

However, Clarissa chose the safe option and married Mr. Dalloway. In the end, she was coddled as a young person and then chose that conventional lifestyle for herself, which was also a choice to to live in a structured, traditionally ordered world -- without passion because it disrupts order and is not "safe." She also chose to live under the thumb of her paternalistic husband, who buys her flowers (a theme (usually for woman and sex: Sally displayed her flowers beautifully but completely non-traditionally) even though Clarissa announces at the beginning of the film, "I'll buy the flowers myself," letting us know right off who she is as a character. Richard is quite fond of her but shows it by giving her chaste kisses on the head, as if she were his child.

Clarissa wonders what would have happened had she chosen a difference path. She asks herself, "Is my life over?" I read that to mean something to the effect of, "Is this all there is?"