Oh, yeah. Used to happen all the time when I was a kid. It seemed at the time as if I lived my whole life erect.
A few times come to mind.
Our class was reading Romeo & Juliet in 10th grade. I'd arranged a date with the girl who was (it turned out) reading ahead of me before the class; I was thinking of all the great and sexy things we were gonna (in my dreams!) do, and popped a boner. That was when Mrs. Knox-Dick (really her name!) decided she wanted to hear me read. I shuffled up to the front of the class (I was a skinny, over-tall geeky kid), but even behind my English book, there was no hiding my hard-on. Jeans and briefs and raging hormones will get you every time! (p.s. We had a great date that Friday!)
Recently, I was standing near the exit in a bus, and scoping out a hot woman who was seated nearby. She looked over to me, and smiled. I smiled back, and started getting hard. Since I was wearing 501s and boxers, my endowment (and probably my circumcision status) became quickly pretty obvious. She kept looking back and smiling, I kept blushing and standing harder.
She got off the bus before me. As she left, she handed me a note with her name and phone number, and whispered, "call me sometime." I did, we started dating, and now we live together when she's in town.