hmmm...interesting question. there's certainly not a specific dress code given for us in the bible, although we are given some general guidance, ie
What does the bible say about showing your stomach?
[bible]1 timothy 2:9[/bible]
In like manner, that women adorn themselves in modest apparel, with shamefastness and sobriety; not with braided hair, and gold or pearls or costly raiment;
modest means different things to different people, and in different times and cultures. i know that a couple of years ago when i first started seeing belly buttons showing, i was pretty mortified

(maybe self-righteously indignant is the right term). i guess the more i saw this style, the less shocking it became to me, but i still don't really like it. i just really don't know.
back when i was a teenager, i had one midriff shirt (that's what short, belly-revealing tops were called back in my day

) and i certainly didn't wear it to be "sexy" or get guys to look at me or anything like that. that truly never crossed my mind. but i was pretty naive back then--well, everyone was more naive back then.
now teens are smacked in the face with sexual messages everywhere, so it's harder for me to believe that teen girls who show their belly (or their thong underwear, or their cleavage, or their whole body by wearing really tight clothing) can do so with the same ignorant naivete that I did 30 years ago. i would hope that today's young christian women would not feel "flattered" by the kind of attention males give them for dressing immodestly. i hope that christian parents are teaching their daughters about purity and the importance of relationships with males that are not all about physical attraction. i don't have daughters, but my sons get the male version of these teachings.