Dan,
there is no "Official" version for N900.
Handset Ux is available for arm and ia32.
There is a Developer Edition for N900 which contains bits and peices from the Handset UX (trunk) and the N900 Hardware Adaption Team's work.
Closed source components include BME, SGX, wireless and some sysinfo stuff specific to the N900.
Current build provides for BASIC telephony and SMS functionality.
Calender (buggy), contacts (V Basic), xterm, wifi, bluetooth etc,picture, music and video (all a bit buggy) etc.
you can make and receive calls and send/receive sms.
Update via xterm using zypper works, but there are some issues woth some packages.
It's a classic alpha/beta release and should not, in any way be seen or assumed to be a "complete" release.
the people working on this are doing a great job and I expect, given time and more community developer input, this will become a viable alternative OS for the N900, and not just for devs/hackers.

Just my .02c