Nokia is rolling out the 1.2009.44-1 update for the N900 over-the-air, if you do not see the 18.5 MB update to Maemo5 in the Application Manager, try refreshing it. If you still do not see the update, it will be because Nokia has not rolled it out in your region yet and you will have to wait.
The 1.2009.44-1 build is only two weeks older than the one the N900 ships with and seems to be a precursor or enabler for the bigger PR 1.1’s OTA update that we can expect soon. This update in itself doesn’t seem to brings much apart from changes to icons of the Application Manager and its behavior after you install an application, now it comes back to the point where you were prior to installing the application.
If you do update, let us know how it went.