If you specify a HTML 4.0 DOCTYPE tag in your master page, the Calendar control will present a couple of issues. First, Appointments in Week View that are longer than a 1/2 hour block, will not expand down to fill the appointment area.  Second, the All Day Event marker in Month View will push past the right border.  Click here to see examples.

I don't have a fix for the first issue. I have tried every hack and workaround that I could think of or find to force the height of the appointment, but to no avail.  I do however have a fix for the second issue.

In your custom CSS file, add the following style:
.ms-cal-workitem .contB, .ms-cal-muworkitem .contB {
 width:95%;
}

This will reduce the width of the All Day Event marker, bringing it back within the constraints of the day borders.