Assume you want to store the locale of user preference in database, which value you will use?
en_US or en-US
They are two standards, but which one you prefer to use as part of your own application?
Updated: Is seems many web sites use dash instead of underscore, e.g.
http://zh.wikipedia.org/zh-tw http://www.google.com.hk/search?hl=zh-TW
I'm pretty sure "-" is the standard. If you see "_" somewhere it's probably something some people came up with to make it a valid identifier.
Personally I'd go with "-", just to be correct.