So some girls wore flip-flops to the White House and older folks are offended, but that's the fashion these days. Flip-flops aren't considered automatically informal by the younger generation, particularly for women. I don't know if you've ever looked at fancy womens shoes, but they're basically flip-flops with heels already, so what's the difference? I've seen people at my work wearing sandals, but they always wear them with socks which is pretty tacky. Still, in a few years I bet sandals will become mainstream enough that "engineer attrire" will include flip-flops. Biggest downside: the air conditioners that keep the computers cold might also lead to uncomfortable feet.
Anyway, if they're good enough for Jesus, they're good enough for me.