When:
1) when MeeGo has a handset version that is released on a phone sold and supported to the general public, and
2) you have checked that all the features you use on your N900 today also will work in this version of MeeGo on an N900, and
3) you're not too dependent of your phone to suffer a full reset and possibly a few days without a phone.

Optional 1.5) ...and you've read the first non-enthusiast reviews.

Is my suggestion.