The topcontainer containing the organization logo, their intranet name and the social notification links broke to two rows in their environment, but not in ours. The cause of this was, I found out, the language specific corev4.css file. In the English (1033) one, the socialnotif width is 120px, but in the Finnish one (1035) it is double! So no wonder. This wasn't how I'd planned the layout!
Problem was of course solved easily as soon as I detected it. But note to self: never overlook the language pack - it changes more than just the language.