Business Or Pleasure?
That's probably the hardest question to answer nowadays for me when crossing the border. My life and my job are pretty intricately mixed nowadays, that it's often hard to differentiate between the two. For example, when I went to WordCamp San Francisco, I was asked that question by the border agent. The conference I was attending definitely interested me on a personal level, especially since I have a personal blog, but clearly I can extract some business value out of it as well. So is it a business trip, or one for pleasure?
The problem with going down the business line is sometimes the border guys get a bit pissy when you eventually tell them you have US clients. Technically that's fine, but sometimes they guys/gals can be a bit anal, and start asking to see contracts or what-not (if the client is near your destination city). My friend Dragos nearly got the anal probe one trip because he was going down to visit Cisco systems but didn't bring a copy of the contract with him.
I typically just say it's a personal trip nowadays, but once they start asking questions about what I do, things always get a bit dicey. I actually find the whole process a bit stressful, and it's ultimately one of the reasons I got my Nexus pass.