I'm also a sophomore (in CS) at Berkeley. My background is a bit similar - I took a year off before college to take a software course and work at a startup. The year I took off was probably one of the periods of greatest personal growth I've ever had in my life. I learned a lot technically, but that wasn't the most important thing I gained (or even the second or third-most important thing). The most important things I gained were friendships and connections with people older than me, and greater perspective on my goals and on college. I like to think I'm a bit more street-smart than I used to be, having been exposed to the real world. It's important to note that taking income minus taxes/expenses, I earned next to nothing over the course of that year (and have a bit of stock that isn't worth much even if the startup continues to do well), so I gained nothing financially.

I don't put much faith in the arguments that "you'll never go back" if you take a year off - I did, and when I made the decision to go back, it was precisely that - a conscious decision, rather than how most students go to college (which is by following the default path). At any time, the decision is yours to return to college - if you decide not to return to school it will only be because you judge your current opportunity (the startup or wherever your career takes you) to be better than going to school.

Obviously the details of this opportunity are pretty important - how much equity are you getting? How much control? Who do you get to work with (the people you meet and/or work with is probably the single greatest factor in your early career)? How much funding do you get? How flexible is the parent company in letting you make decisions (remembering that if you don't have >50% of the equity, they have final say in all decisions)?

If you want to take this offline, my email is in my profile. I wouldn't be so quick to dismiss an opportunity like this. If you think your skills are up to it, it's worth considering. At the very least, I would take the trip to Japan, and come prepared with tough questions for the executives (questions about control, autonomy, flexibility, the conditions of funding, how they make decisions, their philosophy around product management, etc.)