Yo! Gmail Checker v1.2 is a bit short-lived. Version 1.3 is available now (download » here). This is very unusual 'cause I don't have a habit of releasing new versions frequently, so expect to stay with v1.3 for a while ;)

The main new feature is support for Google Apps Email (aka Gmail for Domain). Also, for anyone who prefers the old way of left-click on the statusbar icon to refresh, you can enable it in the Settings window.

Update 1/8:

Version 1.3.1 should fix the problem mentioned here.