I just wanted to note, I was on a plane with my daughter last year and the baby across the aisle was getting formula. They had little formula packets like those packets of individual-serving-size Crystal Light that you dump in your water bottle! So if you have some of those, and you can get bottled water pretty much anywhere, that does simplify the travel formula prep. (obviously you need a baby bottle as well.)
Not that I would choose formula over breastfeeding for travel based on this, but it does seem like feeding formula on the go is getting easier! I thought those packets looked pretty great.