There is no equivalent custom in western culture. It's a way of saying hello, such as a handshake, but it's also a way to appeal or apologize; a sign of respect. It has the additional complication for us that if you wai to someone below your status (like a child or someone of a lower class) it's embarrassing. I found the whole thing interesting because we have no similar custom in western culture, but many asian cultures (thailand, japan) do feature this sort of custom. I suppose it is rooted in our extreme individualism that forms the basis of western culture. We don't like prostrating ourselves or making ourselves vulnerable to another's whims.
I think western culture is weird, or maybe just confused. We want to be individuals and our culture and identity are very self-centric, but we still want everyone to be the same. True individuals scare us. Conversely, I'm beginning to learn that Thai culture, while retaining aspects of eastern, group-focused, respect-laden culture, also is much more freeing and open than western culture. This dichotomy ties into my independent study project. I plan on learning about and (to the extent I can) immersing myself in the Thai GLBT community.
Thailand, and particularly Chiang Mai, has a thriving transsexual community, and there is a history in thailand of men who dress and live as women. And the Thai culture is accepting - it's no big deal to them. They know it is a big deal to westerners though and feel the need to warn western visitors of this aspect of their culture and ask that we respect that they respect their trans population. I think it's fascinating and I'm really excited to spend a semester learning about the community and the culture it's housed in. And I'll get to frequent a wine bar, where one of my profs knows many GLBT people hang out - so all the better!