I just found out, that with the latest syncevolution installed on my Laptop, I can sync to the built-in SyncML-Client of my N900.
I installed syncevolution 1.0.2 from syncevolution.org on my Laptop ( no need for syncevolution on the N900) and configured it with this line:
after that, I could sync with:
My Laptop connected to the phone and synced calendar,addressbook and tasks. I tried to add and delete stuff
and everything gets synced and showed up in Evolution. I didn't know it was THAT easy

what do you have in your laptop? ubuntu?

I use debian.

Quoting the frontpage:

"Binaries are available for Linux desktops (synchronizing data in GNOME Evolution, with KDE supported indirectly already and Akonadi support in development), for MeeGo (formerly Moblin) and for Maemo 5/Nokia N900. The source code can be compiled for Unix-like systems and provides a framework to build custom SyncML clients or servers."

So it's official: there's support for the N900/Maemo!

Syncevolution can be used in two ways:
Either you install it on your N900 and sync to the cloud ( ovi, google etc). This works for quite some time now. But I wanted to sync my
Laptop and didn't want my Data to be somewhere else.
But now you can install syncevolution on your Desktop and sync the built-in client of your N900. Its a little confusing.