I had written a post about missing feature on Gtalk- invisible mode. Last weekend I saw that Gmail Chat added invisible mode feature. It gives flexibility to the user to be in invisible/visible mode.
But this feature wasn’t added to Gtalk which is almost the replica of Gmail chat. So, if a user has logged in to Gtalk and Gmail chat simultaneously then Gmail chat doesn’t allow user to change the mode to invisible mode. I hope soon Google would add this feature to Gtalk also.

