I'm not an English teacher, I am the district coordinator for Career and Technical Student Organizations, so my justification for travel is easy. It's even easier this year because 2914 earned a slot. For me, it's in my basic job description: support our FRC teams.
For PD, you need to be able to tie the talks back into the classroom. You know what your district is pushing as current initiatives, try to tailor to those initiatives. We have fairly new NAF Academies here in DC, and they are really big on industry partnerships. Building relationships with industry is part of the reason we will send teachers to professional conferences as PD. It's important for our teachers to stay relevant with current standards and the direction that industry is headed so our students will be prepared for the workforce. I'll certainly get to talk with industry in St Louis.
The Conference Abstracts are here, just in case you haven't seen them.
http://www.usfirst.org/sites/default...-Abstracts.pdf