This is amazing. The developer, Ross Burton, has posted
“Due to great demand, I've build a package of Contacts 0.5 for the N800.
It's available in the usual place, http://maemo.o-hand.com/.

There are quite some important caveats here:
1) there is no Hildon integration: menus are not part of the titlebar,
the menus have icons, buttons have icons, and so on”

He says that Hildon integration is to follow.

What I did was to download contacts_0.5-2_armel.deb from
http://maemo.o-hand.com/packages/bora/

And install from the file using Application Manager.

It automatically makes the attempt at importing the contacts in the N800 app.