Underscores in apache virtual host names

Dashes != underscores. Dashes are perfectly fine (as long as the name doesn't start or end with them).

Underscores have never been a valid character in hostnames. Apache may've allowed them, but I'd guess it just didn't bother checking for them as opposed to explicitly allowing them.

It's a good thing that it finally starts erroring for this, because you now have to fix it from the very start instead of it biting you in the ass at some point later. I've seen way too many people wondering why their sessions don't work, and it turns out they simply can't because the domain name itself is invalid.

Incidentally, that is also where that popular myth about underscores in cookie names under IE comes from - asked MS support why their cookies don't work, they figured out the user put an underscore in their test hostname - problem solved, but then some other idiot misread that and started telling people that you can't have underscores in cookie names instead of domain names.