hi,
sure the snake can be a phallic symbol,but it is also a healing symbol,and a psychic one .so may be worth looking at from this perspective,
what came to my mind though was the phrase,"a snake in the grass",meaning a hidden enemy,may refer to someone or something you trust, or have trusted in the past that lulled you into a false sense of security,then turned against you,
we should all be on good terms with our inner animal symbols,and if they turn against us ,this is a sign that something is out of balance,
it may well be that it does refer to the sexual aspects,as snakes can represent these instinctive drives,
just a few things to think about,
a lot will depend on your gender,age etc,and to me the most important thing,what the dream is a response to,
hope this helps kick start something for you.
rgds mikey.