You can use the standard BlackBerry multitasking hotkey:
and then press the Esc button on the side. This allows you to multitask on your BlackBerry.
I prefer VeriChat and WebMessenger's much more user-friendly method -- you just hit Esc to force the software to run in the background.
Perhaps the IM+ people can add the feature of being forced to background when hitting the Esc button?