v3.5.5 for N900 is available here.

Installation:
1. Install latest Qt Mobility 1.2 libraries. If you have extras-devel repository enabled you can install those with "apt-get install libqtm-12=1:1.2.0+git20111104-0nd~fremantle5" or if you don't you can download libraries from here (and install with "dpkg -i libqtm-12_1.2.0+git20111104-0nd~fremantle5_armel.deb").
2. Download .deb file from the link above and install it.
3. Copy WMS settings file to "/home/user/MyDocs/.mapsi/" (or attach N900 to computer using mass storage mode and copy settings file to ".mapsi" folder in root (e.g. "D:\.mapsi\") (Create that folder if it don't exist) ) and rename it to "wms_settings.ini".

For latest WMS settings file see this post.