Install xfapplet using synaptic. Add to taskbar. Configure to include modem. Right click to activate.
BTW being able to switch from Ubuntu (GNOME) to Xubuntu (XFCE) using logout, is very handy. Don’t even have to redo the wireless network connection.
[Update] – it wasn’t quite so smooth doing all this with my grandparents’ Dapper Xubuntu setup although I got there in the end. Issue 1 – needed to add a repository with xfapplet in it (it wasn’t there in my grandparent’s Dapper setup). Issue 2 – if you resize the pixel size of the icons on the XFCE taskbar, anything added on the Gnome panel doesn’t resize – and may even become invisible! 🙁 Issue 3 – the taskbar was missing, and even if you ran xfce4-panel to restore it, it disappeared on relogin etc. Solution – looking at setting for sessions. Hid hibernate and sleep and made it automatically save session settings. Success! Phew!