It has appeared for the version 126.96.36.199 of GTalk. The Check for mail button in GTalk menu used to disappear all of sudden and then Voice Mail also gets disabled sometimes! Hence, neither you can check your mail nor you can voice chat. So that button is removed.
Perhaps this problem/feature disable appeared only after the integration of GTalk with Orkut. It has been a while to me for waiting for the new mail message which is displayed at right-bottom of desktop whenever new mail appears – but now, it has been ceized.