It's totally possible that there are companies out there with "unlimited" vacation that would be OK with you taking a month off. However, in my experience, even when you have an assigned number of vacation days there is still pressure to not use them. I can only see unlimited as "don't take more than this person". My boss had told me a few times that he takes vacation days and works on them as if to hint I should be doing the same.

So my thinking of "unlimited" vacation days is in the same way ISPs offer "unlimited" bandwidth :)