Welcome to tPF
frasianpower, a very good point.
It's mostly true but there are different types of synagogues, I doubt anyone would get thrown out of a very progressive one for showing bare arms or shoulders, just a few raised eyebrows but you are totally right for most.
Another thing that this thread does not say is that a Bar Mitzvah (for boys) or Bat Mitzvah (for girls) is not usually just a ceremony, it's a normal Saturday service
plus the Bar or Bat Mitzvah so if in doubt, for the ceremony dress as if going to church including a hat or scarf if you are women (girls can get away with no hat, and don't forget to tell any man to go with to bring something for his head). The ceremony is always on a Saturday and you will be in the temple with others who are there as if in church so always er on the side of conservative.
Then, if there is a seperate party (sometimes on another day) strapless is fine, and shoes to dance in because usually there will be plenty of dancing.