As I've written in https://bugs.maemo.org/show_bug.cgi?id=9544 , Nokia currently has no plans to fix this as the N900 is meant to be an always-online device. Please note that this is not necessarily also my personal opinion.
so I bought the wrong device? even with map downloader and 32G+ storage? even cheapest symbian devices can search in maps offline! and there were predownloaded maps the same way on symbian. also that devices have much lower storage capacity and offline searching is not an issue on them. the storage capacity is simply not a valid reason.
True if Nokia announced/advertised Maps on the N900 as "navigation". Proof welcome.