I'm 36 and I'm just old enough that when I was growing up, visible bra straps were avoided, but then it was fine when I was in college. The people just a few years younger than me are completely comfortable with visible bra straps, and the generation 10 years younger wear their bras as a fashion accessory like Staysfit said.
Since college or so I've been totally fine with visible bra straps under strappy camis & tank tops. I do like it when they coordinate, in terms of color and where the lines fall. But I'll wear a regular bra with a racerback tank top, with no compunction, and I see that on plenty of women around me.
I am trying to become more comfortable with bras as fashion accessories. There are some really cool styles out there that are definitely meant to be seen, in the large armholes of muscle tanks (a look I love) or under sheer shirts. I can't quite bring myself to wear sheer shirts, but I appreciate the look on younger people.
Of course, for work I try to avoid anything that looks that casual or anything that might even hint at being sexy, even if the grad students have no problem with it. I had a meeting with a professor visiting from another university, yesterday, and I felt weird having my bra straps showing under my silk tank top, so I switched the bra straps to racerback to fix that.