Simple answer: Never.

Longer answer:
N900 and its Maemo5 operating system is already abandoned by Nokia and is pretty much dead.
Next device from Nokia, will have Maemo6 operating system which is:
a) very different from Maemo5
b) a dead-end of Maemo operating systems, because we already know it will be discontinued in favor of MeeGo

So... None in the right mind would start developing an application for already dead Maemo5.
And none in the right mind would start developing an application for Maemo6, knowing that it's targeted for death right after seeing the daylight.

They say that we should start developing applications for MeeGo, but no one knows how, because specification is work-in-progress and kept secret.

For me the answer is simple - move to Android.
I ordered Motorola Milestone yesterday.