Yeah, just like bare shoulders and ankles, I think people will get used to it over time.

If you noticed and are distracted (and you're not the only one), I'd argue that it's a them problem not a you problem. If you're "dressed up professionally for an office environment", that dress code is supposed to be boring, conservative, modest clothing. I personally hate workplaces like that, but if that's the kind of place you work, then that's the expectation.

Assuming what you found distracting were nipples, there are ways of not wearing bras while still keeping nipples hidden or at least discrete. If someone's supposed to be "dressed up professionally for an office job", it's reasonable to say that someone who isn't making an effort to hide her nipples isn't meeting the dress code. It would be the same if a guy came in wearing a skin-tight shirt from Father Sons.

I would love it if we lived in a world where workplaces just let people wear the clothing that made them comfortable, but until we reach that world, people who have to dress conservatively for a business environment are going to have to cover up.