I think support for developing Maemo specific Java applications (Hildon bindings...) is not even that important.

But nowadays every high end phone shoulde at least be able to run J2ME applications out of the box.

e.g. for internet banking, since a few months Java based authentication mechnisms are becoming more common - at least here in Germany.

Also digital train/bus tickets, which you can buy via mobile phone are quite popular. These are Java based, too.

Just to name two non-gaming usecases for Java on mobile phones. So if the N900 really doesn't support Java/J2ME this is imo a serious shortcoming, given it's price tag and that it's positioned in the high end segment of the market.