I frequently will get a message on Google Talk. I will drag down the notification bar, click the message, and answer. However, if I then press "Home", and it occurs to me that I wish to send yet another message to this person, I am forced to go find the Google Talk app icon, launch it, select the relevant conversation, and enter my message. Why does Google Talk not appear on the Recent Applications list, accessible by holding down the Home key? Every other app I have does...
Google talk and Gmail each behave this way. I found this out by asking here actually. So when you drag down the notification bar, GMail and Google Talk will not appear in the recent apps list. However, you will see them in recent apps if you navigate to them using an application launcher.
I'm not certain if the same is true for Google Voice.
This is because the Recent Applications list only displays the applications activities. In Android, an Activity is the "window" of the application. Google Talk and GMail always run in background, as a "Service", not an Activity. The application activity is killed by Android when it is not used, and this is the case with those applications, even if they still run in background.