Since last update, Mailplane includes a Google Talk window for an easy chat. Never miss a call again: If somebody wants to talk, Mailplane shows a Growl notification. The brand-new Google Talk Gadget (displayed inside the window) offers some nice features like video and photo preview. See the this blog for more information.
Currently the Google Talk window works with "normal" Google accounts. Support for "Google Apps for your Domain" accounts will be added as soon as the Gadget is ready for it.