I have tried some of the alternative themes from the extras-devel repository and they give a different background for the lock screen.

I'm assuming this works in a similar fashion to themes on a normal Gnome desktop on Linux so it is quite likely that it's easy to change this.

If you're new to Linux then it's handy to differentiate the difference between backgrounds and themes. When switching the background on the N900 all you're really changing is the wallpaper on the desktops. Themes encompass a wider variety of changes to the UI. So for example, buttons, icons and it appears the lock screen.

I haven't got round to building a custom theme for mine yet. As an example here's my current lock screen.