Does the german word for mobile start with 'mobi-'? Or do germans just say 'mobile' like us? if the answer is yes to either of those questions then .mobi will likely be good for germans. in that case, if you've found a great german word then it wouldn't hurt to reg it before someone else does.
just did a quick search on google.de and searched 'mobile' and '.mobi'
from the results that came up it looks like mobile and .mobi are well known in germany.